A senior official from Syria's main opposition group said on Monday that a fragile international attempt to halt nearly five years of fighting was in danger of collapse because of attacks by government forces.

The cessation of hostilities drawn up by Washington and Moscow faced "complete nullification" because Syrian government attacks were violating the agreement, the official of the Saudi-backed opposition High Negotiations Committee (HNC) said.

France said there were reports of attacks on opposition forces in breach of the deal, which came into force on Saturday, and countries backing the Syrian peace process met to try to clarify the situation.

UN Secretary-General Ban Ki-moon said the pause in the fighting was largely holding, despite some incidents that he hoped would be contained. The Kremlin said the process was under way, although it had always been clear it would not be easy.

US Secretary of State John Kerry said efforts were being made to track down alleged violations but that there was currently no evidence to suggest it would destabilize the fragile peace.

The cessation deal does not include jihadist groups such as Islamic State and the al Qaeda-linked Nusra Front and Russia, which is backing the Syrian government with air power, has made clear it intends to keep bombing those groups.

Kerry said at a news conference in Washington that he agreed with Russian Foreign Minister Sergei Lavrov to intensify work on a mechanism to ensure any strikes in Syria solely target Islamic State or Nusra Front.

An aide to Saudi Arabia's defence minister said on Monday, that defence ministers from the US-led coalition against Islamic State had discussed the possibility of a Syrian ground incursion two weeks ago in Brussels.

"It was discussed at the political level but it wasn't discussed as a military mission," Saudi Brigadier General Ahmed Asseri told Reuters. "Once this is organised, and decided how many troops and how they will go and where they will go, we will participate in that."

The cessation of hostilities agreement, the first of its kind since the Syrian civil war began in 2011, is a less formal arrangement than a ceasefire. It is meant to allow peace talks to resume and aid to reach besieged communities.

The British-based Syrian Observatory for Human Rights said it was largely holding, with casualties greatly reduced compared with before the agreement took effect.

But Syrian forces made some gains. The Observatory reported they had taken territory near Damascus on Monday after a battle with the Nusra Front and other Islamist rebels.

Syrian government forces also regained control of a road to the northern city of Aleppo after making advances against Islamic State fighters.

Aid trucks carrying non-food items such as blankets on Monday entered Mouadamiya, a suburb of Damascus under siege by government forces, the Syrian Arab Red Crescent said.

The United Nations and other agencies hope to deliver aid to more than 150,000 people in besieged areas over the next five days.

Kerry said he was concerned by reports the Syrian government was creating obstacles for the delivery of humanitarian aid and hoped it would stop its officials and troops from taking medicine or other supplies from the shipments.

Gloomy assessment

Asaad al-Zoubi, head of the HNC's delegation to the peace talks, gave a gloomy assessment of the truce. "We are not facing a violation of the truce...we are facing a complete nullification," he said on Al Arabiya al Hadath TV.

"I believe the international community has totally failed in all its experiments, and must take real, practical measures towards the [Syrian] regime," Zoubi said, without elaborating.

He said there were no signs of any preparations for peace talks, which the United Nations wants to reconvene on March 7.

Talks in Geneva in early February collapsed before they started, with rebels saying they could not negotiate while they were being bombed.

HNC spokesman Salim al-Muslat said the truce was a step in the right direction, but a mechanism was needed to stop such violations and encourage negotiations.

"There has to be a power that really stops what Russia and what the regime is doing," Muslat said in a television interview with Reuters in Riyadh. "Today there [were] about 10 Russian air strikes, about 16 air strikes done by the regime."

Syrian officials could not immediately be reached for comment on allegations that government forces were violating the cessation. The government has said it is abiding by the agreement.

But a Syrian Foreign Ministry official accused Saudi Arabia of trying to undermine the cessation of hostilities agreement by saying there would be a 'Plan B' if it failed. He did not give details of the plan, which is believed to include military action.

Russia on Monday also rejected any suggestion of a Plan B, which has been alluded to by Kerry.

Countries belonging to the 'International Syria Support Group' (ISSG), led by the United States and Russia, met in Geneva on Monday. They are supposed to monitor compliance with the deal and act rapidly to end any flare-ups.

"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 said in Geneva. "All this needs to be verified."

Air strikes are heavy

The HNC said the cessation of hostilities was broken by the Syrian government 15 times on the first day, and that there were further violations by Russia and Hezbollah, both allies of President Bashar al-Assad.

On the ground, rebels said the violence was below pre-ceasefire levels in some places and little changed in others.

Colonel Fares al-Bayoush, head of a Free Syrian Army group called the Northern Division, told Reuters: "The air strikes are heavy today, especially by Russian planes."

Abu al-Baraa al-Hamawi, a fighter with the Ajnad al-Sham group in northwestern Syria, said the government had shelled a number of villages. "It is regular bombardment, no change. The regime after the truce is as it was before."

The Syrian Observatory for Human Rights, which monitors the war through a network of contacts on the ground, said the number of people dying each day had gone down substantially since the cessation started.