KARKAMIS/ankara - Turkey on Tuesday sent more tanks into Syria and sternly warned a Kurdish militia to withdraw from frontline positions, a day after pro-Ankara Syrian opposition fighters captured a key border town from militants.

The tanks joined those which crossed the frontier on Wednesday in the so-called Operation Euphrates Shield, which Turkey says aims at ridding the northern Syrian border area of both Islamic State (IS) extremists and Kurdish militia.

Turkish President Recep Tayyip Erdogan said on Wednesday that the offensive had expelled IS from the Syrian town of Jarabulus, and pro-Ankara rebels reported the militants had retreated south to the town of Al-Bab.

But Defence Minister Fikri Isik warned the Kurdish People’s Protection Units (YPG) militia - who also had designs on Jarabulus - to move back east across the Euphrates or also face intervention from Turkey.

The new contingent of tanks roared across a dirt road west of the Turkish border town of Karkamis, throwing up a cloud of dust in their wake before crossing the border, an AFP photographer said.

They were then followed by around 10 armoured vehicles. The operation, the most ambitious launched by Turkey during the five-and-a-half-year Syria conflict, has seen Turkish special forces deployed on the ground and jet fighters striking IS targets.

They are supporting a ground offensive by hundreds of Syrian rebels who on Wednesday marched into Jarabulus and a neighbouring village after meeting little resistance.

It was not immediately clear if the deployment of the new tanks on Thursday was aimed at securing Jarabulus or helping the rebels move into new territory. But a Turkish official said on Wednesday that Ankara would “continue operations until we are convinced that imminent threats against the country’s national security have been neutralised.”

Turkey is prepared to stay in Syria for as long as it takes to destroy Islamic State, US Vice President Joe Biden said during a visit to Sweden a day after meeting Turkey’s President.

“I think the Turks are prepared to stay in an effort to take out ISIL as long as it takes,” Biden told reporters, using an acronym for the militant group. “I think there has been a gradual mind shift ... in Turkey, with the realization that ISIL is an existential threat to Turkey.”

Eleven children were killed on Thursday in a barrel bomb attack carried out by government forces on a rebel-held neighbourhood of Syria’s Aleppo city, a monitor said.

“Fifteen civilians, among them 11 children, were killed in a barrel bomb attack on the Bab al-Nayrab neighbourhood” in southern Aleppo city, the Britain-based Syrian Observatory for Human Rights monitor said.

Russia has agreed to a 48-hour humanitarian ceasefire in the divided Syrian city of Aleppo to allow aid deliveries, but security guarantees are awaited from other parties on the ground, UN officials said on Thursday.

The United Nations has been pushing for a weekly 48-hour pause in fighting in Aleppo to alleviate suffering for some 2 million people, with major powers backing opposing sides in Syria’s five-year-old civil war.

“We have...agreement now from the Russian Federation for the 48-hour pause, we’re waiting (for) it from the other actors on the ground. That has taken more time frankly than I thought was needed,” said Jan Egeland, who chairs the weekly UN humanitarian task force, told a news briefing in Geneva.

China’s military will provide training for Syrian armed forces, a spokesman for Beijing’s defence ministry said Thursday, adding it would take place on Chinese soil.

“The Chinese military will provide the Syrian side with medical and nursing professional training,” defence ministry spokesman Wu Qian told reporters at a monthly briefing. The training would take place in China, he added, and was intended “to ease the humanitarian crisis in Syria”.

PKK blamed for deadly attack on opposition chief's convoy

One Turkish soldier was killed on Thursday when Kurdish militants opened fire on the convoy of Republican People's Party (CHP) leader Kemal Kilicdaroglu but the opposition chief was unharmed.

Kilicdaroglu's convoy was driving through the northeastern region of Artvin when it came under attack from nearby woods.

"Terrorists fired on members of the gendarmerie who were protecting the convoy," Interior Minister Efkan Ala said in televised comments.

Ala said three soldiers were wounded. But the state-run Anadolu news agency later said one had died of his wounds.

The minister blamed the ambush on members of the outlawed Kurdistan Workers' Party (PKK) and said an operation had been launched against the rebels in the area.

Kilicdaroglu said he was safe and well after the incident. "There is no problem, we are fine, don't worry," he told CNN-Turk television by telephone.

He later sent an emotional tweet: "We only have one God-given life, so let it be sacrificed for this homeland, this nation, this people!"

- 'Won't damage unity' -

President Recep Tayyip Erdogan and Prime Minister Binali Yildirim called Kilicdaroglu to offer their best wishes, with television pictures showing the shaken opposition chief taking the call from the president on his mobile phone.

Since the failed July 15 putsch, there has been greater harmony between the opposition and the government, with Kilicdaroglu meeting Erdogan and Yildirim several times.

Kilicdaroglu and other opposition leaders strongly and rapidly condemned the coup bid, winning plaudits from the government for their conduct.

Yildirim said the attack would not damage Turkey's unity, vowing that the "terrorists" would be punished.

"These terrorists will not damage the unity within Turkey... or achieve their ambitions. They will face the punishment they deserve."

Adding to the show of unity, Nationalist Movement Party (MHP) leader Devlet Bahceli offered his best wishes on Twitter.

"To Mr Kilicdaroglu and the Republican People's Party community, I give my best wishes. We strongly condemn this terror attack."

The co-chairs of the pro-Kurdish Peoples' Democratic Party (HDP) -- Figen Yuksekdag and Selahattin Demirtas -- also telephoned Kilicdaroglu to wish him well and condemn the attack, the party said.

PKK militants have on occasion staged attacks in the northeastern area close to the Black Sea although it is outside their traditional region of operation.

Over 40,000 people have been killed since the PKK first took up arms in 1984 with the aim of carving out an independent state for Turkey's Kurdish minority, although now it focuses more on rights and demands for greater autonomy.

It is proscribed as a terrorist group by Turkey, the European Union and the United States.

The HDP has so far been frozen out of the post-coup national unity talks, with the government accusing it of failing to break links with the PKK.

The well-connected columnist of the Hurriyet daily, Abdulkadir Selvi, said the aims of the operation included creating a security zone free of “terror groups” and limiting the advances of Kurdish militia.

He said 450 members of the Turkish military had been on the ground on the first day of the offensive but this number could rise to 15,000.

The Hurriyet daily, citing military sources, said 100 IS militants had been killed in the offensive. It is not possible to independently verify the toll. State-run news agency Anadolu said one rebel fighter was killed but the Turkish armed forces sustained no losses.

Jarabulus, a small town on the west bank of the Euphrates a couple of kilometres (miles) south of the border, had been held by IS militants since the summer of 2013.

Turkey has emphasised that the offensive was also aimed at the YPG, who Ankara sees as a terror group bent on carving out an autonomous region in Syria.

Ankara’s hostility to the YPG puts it at loggerheads with its NATO ally, the United States, which works with the group on the ground in the fight against IS. But US Vice President Joe Biden, visiting Turkey on Wednesday, made clear that Washington has strictly told the YPG not to move west of the Euphrates after recent advances or risk losing American support.

Turkish Defence Minister Fikri Isik told NTV television there was so far no evidence of any withdrawal and Turkey reserved the right to strike the YPG if it failed to move. “If this withdrawal doesn’t happen, Turkey has every right to intervene,” Isik added.

“They have not yet withdrawn... Turkey will be following, moment by moment,” Isik said, adding the withdrawal was promised within a week.

A spokesman for the US-led coalition against IS tweeted that the “main element” of the Syrian Kurdish forces had already moved east although some remained for clean-up operations.

Ankara has in the past been accused of turning a blind eye to the rise of IS but hardened its line in the wake of a string of attacks - the latest a weekend bombing on a Kurdish wedding in the city of Gaziantep that left 54 people dead, many of them children.

The Jarabulus operation proceeded at lightning speed with the town captured from IS just 14 hours after it was launched.

The speed of the advance stood in stark contrast to the long, grinding battles it had taken for Kurdish forces to recapture towns from IS in northern Syria, such as Kobane and Manbij.

Television pictures showed the Syrian fighters walking into an apparently deserted and abandoned Jarabulus unchallenged and newspapers published pictures showing that the rebels even had time to take selfies along the way.

The apparent efficiency of the operation also marked a major boost for the Turkish army whose reputation had been badly tarnished by the failed July 15 coup against Erdogan staged by rogue elements in the armed forces.