ISTANBUL/WASHINGTON - Turkey on Tuesday pounded Islamic State (IS) militants in Syria with new artillery strikes as expectations grew of a major Ankara-backed offensive against the group after a deadly suicide bombing on its soil.

With tensions flaring on the Turkey-Syria border following the bombing in the nearby city of Gaziantep that left 54 people dead, Turkish howitzers on Monday hit militant and Kurdish rebel targets across the frontier. Turkey has been shaken by one of the bloodiest years in its modern history, with a string of attacks by IS militants and Kurdish militants and the botched July 15 coup.

In new fighting on Tuesday, two mortar rounds fired from an IS-controlled area in Syria hit the southeastern Turkish town of Karkamis while three more hit the centre of the Turkish border town of Kilis, the state-run Anadolu news agency said. There were no reports of injuries although 21 people in Kilis have been killed by fire from Syria in recent months. Turkish artillery responded to the fire on Karkamis by hitting four IS positions around the militant-controlled Syrian town of Jarablus with around 60 shells. The army also responded to the fire on Kilis. The shelling came as activists said hundreds of Ankara-backed rebels were preparing an offensive against the IS group to seize control of Jarablus. But this could potentially put them on collision course with the militia of the Kurdish Democratic Union Party (PYD) which Ankara vehemently opposes and who also have designs on Jarablus after seizing the strategic Manbij area in northern Syria from IS.

Rami Abdul Rahman, the head of the Syrian Observatory for Human Rights, said the “Turkish shelling in Syria aimed to prevent the advance of troops backed by Kurds from Manbij towards Jarablus”.

US deploys attack helicopters to strike IS in Libya

 The United States has begun using attack helicopters to target the Islamic State group in the jihadists' Libyan former stronghold of Sirte, a defense official said Tuesday.

American jets have since the start of August been helping fighters loyal to the Government of National Accord (GNA) retake the coastal city of Sirte, conducting dozens of strikes on IS fighting positions and equipment.

Lieutenant Commander Anthony Falvo, a spokesman for the US military's Africa Command, said Marine Corps AH-1W SuperCobra attack helicopters had in recent days joined the operation. "The Cobras provide additional precision air strike capability," Stuttgart-based Falvo told AFP in a phone interview. "It gives us a little bit more flexibility."

SuperCobra gunships are based on the USS Wasp, an amphibious assault ship in the Mediterranean. Harrier jets are also being launched from the ship to conduct strikes on Sirte.

As of Monday, the United States had conducted 77 air strikes in the city. Pro-GNA forces, backed since August 1 by US air strikes, began an assault in mid-May to expel IS from what had been its Sirte stronghold.

The jihadists seized control of the city, which had been the hometown of Libya's slain dictator Moamer Kadhafi, in June 2015.

They fought their way into Sirte on June 9 and have since faced a barrage of sniper fire, suicide bombings and booby traps, pinning down the jihadists in a downtown area near the sea.

More than 350 pro-GNA fighters have been killed and nearly 2,000 wounded in the battle, according to medical sources. IS casualty figures are unavailable.

The Pentagon has said US involvement in the Sirte operation would last "weeks not months," but Salvo did not want to speculate on how much longer the United States would continue its air campaign.

"We are there at the request of the GNA," he said.

"We will continue our support for as long as it is requested. If they were to tell us tomorrow they don't need our support any more, we would end our support at that point."

He told AFP the commander of Kurdish-dominated forces headed to Jarablus, Abdel Satar al-Jader, was also “assassinated” on Monday after announcing he planned to resist the Turkish advance. There was no confirmation of this from Turkish sources.

Abdulkadir Selvi, a well-connected columnist for the Hurriyet daily, said the Turkey-backed offensive “could begin at any moment”. The movements have come at a critical juncture for Turkey in Syria’s five-and-a-half-year war, with signs growing it is on the verge of a landmark policy shift. Ankara has always called for the ouster of President Bashar al-Assad as the key to ending the conflict, putting Turkey at odds with his main supporters Iran and Russia. However Prime Minister Binali Yildirim at the weekend for the first time acknowledged that Assad was one of the “actors” in Syria and may need to stay on as part of a transition.

On Monday, he urged world powers including Iran, Russia and the United States to join together to rapidly open a “new page” in the Syria crisis. But he also warned it was “unacceptable” for Kurds to seek to establish any kind of separate entity in northern Syria.

Turkey regards PYD as a terror group, although Washington, Ankara’s ally in the fight against IS, sees its People’s Protection Units militia (YPG) - as having an important role in the fight.

US Vice President Joe Biden is due to visit Ankara Wednesday to meet Turkey’s leadership with agreeing a unified strategy on Syria set to be a crucial issue.

Turkey had long been accused of turning a blind eye to the rise of IS - charges it vehemently denies - but has taken a tougher line following attacks at home. It has lately been building a 240-kilometre (150-mile) concrete wall in order to tighten security.

The attack in Gaziantep on a Kurdish wedding party for a young couple has horrified the country, with the majority of the 54 victims aged under 18 and including children as young as four. But there is confusion as to who was behind the attack, with President Recep Tayyip Erdogan initially saying the suicide bomber was a child aged 12-14 acting on the orders of IS.

However Yildirim on Monday said Turkey still had no clue who carried out the attack and said all “rumours” over the age and affiliation of the bomber should be taken with a pinch of salt.

Hurriyet said the authorities still suspected IS was behind the attack as the main line of inquiry, with investigators taking DNA samples in Gaziantep from the families of possible IS suspects.

Reports said the previous assertions by the authorities had been based on eyewitness statements rather than concrete scientific evidence.

One guest at the wedding party, Emine Ayhan, lost four of her five children in the blast, Turkish media reported.

“To finish off the evening the young guys wanted one last song. It was in the middle of this song that the bomb went off,” Hurriyet quoted her as saying.