By Tom Perry and Tom Miles
BEIRUT/GENEVA (Reuters) - A Syrian or Russian air strike was reported to have killed at least 19 people and possibly many more at a market in northwestern Syria on Monday, straining a cessation of hostilities agreement meant to pave the way for peace talks.
In a further upsurge in violence, al Qaeda's Nusra Front and other Islamist insurgents not included in the U.S.-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.
Rebels have said government forces, their war effort buoyed by five months of Russian air strikes, appear to be mobilizing forces. Opposition fighters say there have been numerous government attacks on their positions during the cessation, notably in northwestern Syria near the border with Turkey.
Russia has meanwhile said weapons are being supplied daily to rebels from Turkey, a major foreign sponsor of the rebellion.
Russia's Defence Ministry said on Monday eight ceasefire violations had been registered in Syria over the past 24 hours.
The Syrian army has said very little about operations in western areas of Syria covered by the agreement, though it has said operations against Nusra continue.
The town struck in Monday's air strike is close to an air base which Nusra Front and other groups captured last September. Government forces also shelled the rebel-held town of Jisr al-Shughour in Idlib province, the Observatory said.
Hijab, speaking in a conference call with reporters, said he had sent a letter to U.N. Secretary General Ban Ki-moon to complain about a map of armed groups' positions, which was published by the Russian Defence Ministry. He said the map was not accurate.
In a separate incident, the Syrian Kurdish YPG militia said the death toll from insurgent attacks on a mainly YPG-controlled residential quarter of northern city Aleppo on Sunday had risen to 16, including nine children.
The Observatory said it was the biggest single toll since the agreement came into effect.
The YPG said later on Monday insurgents from the Failaq al-Sham Islamist group had "infiltrated" near a village near the YPG-controlled city of Afrin in northwestern Aleppo province and fired at residents.
The YPG is backed by the United States to fight Islamic State in Syria's northeast, and has been the most effective partner on the ground for a U.S.-led air campaign against IS in Syria.
It has separately been battling insurgents that receive support through Turkey, to the alarm of U.S. ally Ankara, which sees the Kurdish fighters as a danger along its border and has shelled the YPG in the Afrin area.
Western states have said the cessation of hostilities appears to be largely holding, hoping that will allow for peace talks to get underway. A previous attempt to convene talks was aborted in February before any face-to-face meetings took place.
The obstacles to talks remain formidable, including differences over the future of Assad. The opposition wants Assad removed from power at the start of a transitional period, a demand Western countries have backed away from as Russia's military intervention has reshaped the war in his favor.
HNC member Riad Nassan Agha said a final decision on attending the Geneva talks would depend on issues including the degree of compliance with the truce and progress toward easing humanitarian conditions.
But noting what he described as a reduction in ceasefire violations by the government side in the last two days, Agha said "our inclination is to go" and said he expected opposition delegates to start arriving on Friday.
He added that truce violations must be reduced to "zero" and that nothing else must happen to obstruct the start of talks.
"We will go, God willing," he said.
The agenda must focus on the "formation of a transitional governing authority" in line with a U.N. Security Council resolution, he said, adding: "We will not accept getting into issues outside what the resolution sets out".
(Additional reporting by John Davision; Editing by Dominic Evans, Philippa Fletcher, Peter Graff and Andrew Hay)