The leaders of Syria's opposition delegation were leaving Geneva on Tuesday after the group suspended its formal participation in troubled peace negotiations, the opposition chief said.

The coordinator of the main opposition High Negotiations Committee (HNC), Riyad Hijab, told journalists he was leaving a day after the group put its participation at the talks on hold to protest escalating violence and restrictions on humanitarian access in Syria.

Suspected government air strikes on Tuesday killed at least 44 civilians at two markets in a part of northwestern Syria controlled by the war-torn country's Al-Qaida affiliate, a monitor said.

The Syrian Observatory for Human Rights described as a "massacre" the strikes on a vegetable market in Maaret al-Numan that killed at least 37 civilians and at least another seven at a fish market in nearby Kafranbel.

French President Francois Hollande said Tuesday that the Syrian opposition's decision to suspend participation in peace talks was "worrying", as he offered to support Jordan in dealing with refugees fleeing the five-year conflict.

Speaking a day after Syria's main opposition announced its formal participation in peace negotiations in Geneva was on hold, Hollande said he was concerned that a ceasefire that has dramatically reduced fighting across Syria might not last.

Syria's regime is prepared to discuss the creation of a new unity government at peace talks in Geneva but President Bashar Assad's fate remains off limits, its lead negotiator told AFP Tuesday.

Assad's chief representative in Geneva, U.N. ambassador Bashar al-Jafaari, had during previous rounds of peace talks insisted that any discussion of a political transition in Syria was premature.

Around 50,000 Syrians are stranded in no-man's land along the border with Jordan, a three-fold increase since January because of increased security checks, state news agency Petra reported Monday.

The agency, quoting government spokesman Mohamed Momani, said the estimated 50,000 refugees were massed around the desert border posts of Hadalat and Rokban.

Many Syrian refugees in Lebanon say their lives have ground to a halt since new measures made it almost impossible for them to obtain or renew their residence permits.

More than half of Syrian refugees in Lebanon do not have valid permits, according to the United Nations, leading to a rising number of newborns going unregistered.

Syria's Al-Qaida affiliate and allied rebels pushed offensives around northern, central and coastal Syria on Monday, triggering a spike in violence that could threaten a truce ahead of peace talks, a monitoring group said.

The Islamic State (IS) group also took back control of the town of Al-Rai near Turkey, which rival rebels had captured last week, the Syrian Observatory for Human Rights said.

Bassam Dabbas did not think he would survive to see his hometown of Al-Qaryatain retaken when Islamic State group jihadists seized control and captured him and hundreds of other Christians.

Now, around eight months later, he stands in the charred remains of the Mar Elian church where he once used to pray, and struggles to digest that he has returned to see government forces in charge.

A U.S. citizen who was held in Syria has been freed by Bashar Assad's regime, the State Department said Friday, after a report that a missing journalist is coming home.

"We can confirm and welcome the news that a U.S. citizen was released by Syrian authorities," State Department spokesman John Kirby said.

Syrian rebels seized control Thursday of the Islamic State group's main supply route to Turkey, a monitor said.

"Rebel factions and Islamists took control of the northeast of Al-Rai," a town occupied by IS on the border between Syria and Turkey, the Syrian Observatory for Human Rights said.