TEHRAN/DAMASCUS - An Iranian general has been killed by jihadists in Syria, reports say, the latest high-profile casualty of Tehran’s efforts to prop up President Bashar al-Assad.

Brigadier General Mohsen Ghajarian of the elite Revolutionary Guards was killed in the northern province of Aleppo, according to the Fars news agency, which is close to the Guards.

He was advising pro-government forces in the fight against the Islamic State (IS) jihadist group, it reported, without saying when he died.

MizanOnline, a news agency linked to the judiciary, also reported his death. Fars said six Iranian “volunteers” had also been killed in Aleppo province, where the Syrian army, backed by Russian air strikes, this week launched a major offensive against rebels.

Iran has provided military advisers to Assad’s army, as well as Iranian, Afghan, Iraqi and Pakistani “volunteers” to fight the rebels.

The latest deaths bring to more than 100 the number of advisers and “volunteers” sent by Iran who have been killed in Syria since the beginning of October, according to an AFP tally.

The casualties include General Hossein Hamedani, a veteran of the Iran-Iraq war who died in October after having conducted 80 missions in Syria.

Assad’s forces Thursday entered the villages of Nubol and Zahraa north of Aleppo city, after breaking a rebel siege there, prompting what Syrian state news agency SANA called “mass celebrations” in the streets.

Iranian state media was also celebrating the victory with live reports from the battlefront. The conflict has killed more than 260,000 people and forced half the country’s people from their homes since March 2011.

In London, US Secretary of State John Kerry demanded Thursday that Russia stop bombing the Syrian opposition, implicitly blaming Moscow for the collapse in peace talks.

Speaking in London ahead of a conference on the Syrian humanitarian effort, Kerry said he had called Russian Foreign Minister Sergei Lavrov for a “robust” discussion.

In Geneva on Wednesday, UN envoy Staffan de Mistura suspended attempts to begin a dialogue between Bashar al-Assad’s regime and the Syrian opposition. Alongside Britain’s Foreign Minister Phillip Hammond, Kerry read out sections of UN Security Council resolution 2254, passed in December, calling for an immediate ceasefire. “Russia has a responsibility, as do all parties, to live up to it,” he said.

“So I had a conversation this morning with Foreign Minister Lavrov. We discussed, and we agreed, that we need to discuss how to implement the ceasefire.”

In Syria, the government troops moved closer on Thursday to encircling rebels in the country’s second city Aleppo, threatening a total siege after cutting their main supply line.

Backed by a wave of Russian air strikes, regime forces have made key advances against rebel positions on the road to Aleppo and the offensive has been blamed for causing the suspension of peace talks this week in Geneva.

At least 21 civilians, including three children, were killed Thursday in Russian strikes on rebel-held districts of Syria’s Aleppo city, a monitor said. The Britain-based Syrian Observatory for Human Rights said the strikes on six neighbourhoods of the northern city had also wounded many, and that the toll could rise.

The advance - which has seen the most significant government victories since staunch ally Russia launched air strikes last year - brings government forces closer than ever to encircling rebels who have held the east of Aleppo city since mid-2012.

Since it began Monday, the offensive has seen troops sever the main rebel supply route from Aleppo city to the Turkish border, allowing them to also end a long-running opposition siege on two Shia towns on the same road, Nubol and Zahraa.

Government soldiers and pro-regime militants arrived in the towns on Thursday morning to cheering crowds who threw rice and ululated, according to footage shown on state television.

Syria’s state news agency SANA reported “mass celebrations in the streets of Nubol and Zahraa welcoming army troops and celebrating the breaking of the siege.”

State television showed residents embracing arriving fighters, who fired into the air and waved the flags of the Syrian government and the Lebanese Hezbollah, a crucial regime ally.

Hezbollah’s Al-Manar television station also broadcast footage from inside the two towns, showing residents chanting slogans in support of President Bashar al-Assad and Hezbollah chief Hassan Nasrallah.

The two towns, home to about 35,000 people before the war, had been besieged by rebels since 2012, and reaching them had long been a government goal.

But an even higher priority has been severing rebel supply routes into Aleppo city, which has been divided between opposition control in the east and government control in the west since shortly after fighting there began in mid-2012.

Last February, government forces launched a similar offensive north of the city which stalled shortly after it began.

But this operation was backed by heavy air strikes from Russian forces, who intervened in the conflict to bolster Assad’s government in late September.

The Syrian Observatory for Human Rights, a Britain-based monitor, said Moscow’s warplanes had launched hundreds of strikes since the offensive began.

The group said some 100 rebel fighters had been killed in clashes and the strikes, along with 64 regime troops.

The government advances leave the rebels in eastern Aleppo city surrounded from the south, east, and north, with only a single opening to the northwest leading to the neighbouring opposition-held province of Idlib.

A range of opposition groups are present in the region, including moderate rebels, Islamists and the Al-Qaeda affiliate Al-Nusra Front.

Observatory chief Rami Abdel Rahman said the opposition in Aleppo now risked being completely encircled.

“They have lost their main supply route to the Turkish border; the only remaining route is long and complicated, and will be the regime’s next target,” he said.