United Nations Secretary-General Ban Ki-moon said on Monday that a cessation of hostilities in Syria was largely holding but that major and regional powers were looking into some incidents that he hoped would be contained.
"By and large the cessation of hostilities is holding, even though we have experienced some incidents," Ban told reporters in Geneva after talks with his envoy Staffan de Mistura and before a meeting of the International Syria Support Group (ISSG).
"But the task force and all other members of this ISSG are now trying to make sure that this does not spread any further and this cessation of hostilities can continue."
However, a Syrian opposition official said a fragile cessation of hostilities that took force early on Saturday faced "complete nullification" because Syrian regime attacks violated the agreement drawn up by the United States and Russia.
A spokesman for the Saudi-backed opposition High Negotiations Committee (HNC) said the cessation of hostilities was broken by Syria's regime 15 times within the first day, and that there were further violations by Russia and Hezbollah, both allies of regime leader Bashar al-Assad.
Countries backing the Syria peace process will meet at 3 pm (1400 GMT) in Geneva on Monday as France demanded information about attacks in breach of a cessation of hostilities.
"We have received indications that attacks, including by air, have been continuing against zones controlled by the moderate opposition," French Foreign Minister Jean-Marc Ayrault told reporters at the UN Human Rights Council in Geneva.
"All this needs to be verified. France has therefore demanded that the task force charged with overseeing the cessation of hostilities meet without delay," Ayrault said.
The United Nations hopes the cessation of hostilities agreement, which is less binding than a formal ceasefire and was not directly signed by the Syrian warring sides, can precede a more formal ceasefire.
The plan does not apply to DAESH and the Al Qaeda affiliate Nusra Front, which Moscow and the Syrian regime have said they will continue to target. The opposition fears they will use this as a pretext to target moderate opposition groups.
The UN estimates demonstrate that there are almost 500,000 people living under siege, out of a total of 4.6 million, who are living in difficult conditions to receive aid, but it hopes that the current cessation of hostilities will bring an end to the 15 sieges.