Air strike hits Syrian market, opposition says truce must be respected

In a further upsurge in violence, al Qaeda's Nusra Front and other Islamist insurgents not included in the US-Russian agreement attacked government forces in a neighboring province, taking over a village and at least two hilltops in their first advance for some time in the area, a monitoring group said.

The agreement, accepted by President Bashar al-Assad's government and most of his enemies, has reduced violence in Syria since it took effect on Feb 27, the first truce of its kind in a 5-year-old war that has killed more than 250,000 people and caused the world's worst refugee crisis.

Foreign powers hope the pause in fighting can lead to peace talks to end the conflict. But the agreement, which has not been directly signed by the Syrian warring parties and is less binding than a formal ceasefire, is very fragile and each side has accused the other of breaking it.

Damascus and Moscow have vowed to continue fighting groups outside the agreement such as Islamic State and the Nusra Front, which is widely deployed across western Syria in close proximity to groups that agreed to cease fire. Many rebels say they believe the government and its Russian allies can use the presence of the militants as an excuse to fight on.

The death toll from the air strike on a market selling diesel in rebel-held Idlib province was likely to rise, the Syrian Observatory for Human Rights monitoring group said, adding that it did not know whether the Syrian government or its Russian ally was responsible.

Riad Hijab, chairman of the opposition High Negotiations Committee (HNC), said "tens" of people had been killed in what he described as a massacre. There was no word from the Syrian government, which has said it is respecting the agreement.

Hijab said the opposition would decide by the end of the week whether to attend the talks, which the United Nations aims to start this week. Another HNC member told Reuters it was leaning toward going.

The Nusra Front and the Islamist Jund al-Aqsa attacked government forces in southern Aleppo province and captured the village of al-Ais and two nearby hilltops in subsequent clashes, the Britain-based Observatory said. There were many casualties, it said, without giving an exact figure.

It was the first advance by Nusra Front in the area this year, the Observatory said. Nusra said in online statements it had taken over the same hilltops.

Russia's intervention in the war has in recent months helped Syrian government forces and their allies recapture territory lost last year in many parts of western Syria, including Aleppo province. Insurgents have made some gains elsewhere, such as Hama province.

The cessation of hostilities agreement has been followed by more aid deliveries to opposition-held areas blockaded by the government, though the opposition says the quantities fall far short of needs.