Moscow: The situation in Syria is deteriorating, the Russian military said Saturday, blaming rebels for stepping up attacks and saying the United States would be responsible if the current ceasefire broke down.

"The situation in Syria is worsening," said Russian General Vladimir Savchenko in a briefing shown on television.

"In the past 24 hours, the number of attacks have risen sharply," with 55 attacks on government positions and civilians.

He said 12 civilians had died in the attacks, including two children and a Syrian Red Crescent volunteer.

Russian military officials lashed out at the United States in the strongest language yet over the ceasefire struck last week in Geneva, a last-ditch effort to stop the bloodshed in Syria. The ceasefire has so far lasted five days.

"Russia is exerting all possible effort to restrain government troops from returning fire," senior army general Viktor Poznikhir said during the televised briefing.

"If the American side does not take the necessary measures to carry out its obligations... a breakdown of the ceasefire will be on the United States."

Russian President Vladimir Putin accused Syrian rebels Saturday of using the ceasefire to regroup, as diplomatic tensions between Moscow and Washington simmered over a lack of humanitarian aid access.

Fresh shelling and clashes were reported overnight in some areas of the war-torn country, but the US-Russia brokered truce which took effect on Monday appeared to be largely holding.In New York, the UN Security Council cancelled an urgent meeting that had been called to discuss whether to endorse the ceasefire, billed as the "last chance" to end the five-year war that has killed 300,000 people.

The closed-door consultations were scrapped after Moscow and Washington failed to agree over disclosing details of the ceasefire to the council.

Putin, a key ally of Syrian President Bashar al-Assad, said he remained "positive" about the truce but lashed out at rebels.

"We see attempts to regroup among these terrorists, to switch one label for another, one name for another and keep their military capacity," he said in televised remarks while on a trip to Kyrgyzstan.

Putin said Washington apparently "has the desire to keep the capabilities to fight the lawful government of President Assad," calling it a "very dangerous path."

Moscow said Friday that it was ready to prolong the truce by 72 hours, but there has been no formal announcement of an extension.

The implementation of the truce has been complicated by the presence of jihadists -- who are not covered by the ceasefire -- and mainstream rebels on some of the same frontlines.

A challenge for Washington is to persuade opposition groups it backs to separate themselves from the former Al-Qaeda affiliate Fateh al-Sham Front, previously called Al-Nusra Front.

US Secretary of State John Kerry called his Russian counterpart Sergei Lavrov on Saturday and was urged to press the rebels for safe passage guarantees the Russian foreign ministry said.A key plank of the truce deal was the delivery of aid to areas including Aleppo, where an estimated 250,000 people in rebel-held areas of the city are living under government siege.

Under the deal, the main route into Aleppo, the Castello Road, would be demilitarised and aid convoys would enter from Turkey.

But 40 trucks carrying desperately needed food aid were still stuck on the border with Turkey on Saturday.

"Still no progress, but the UN is ready to move once we get the go ahead," said David Swanson, a spokesman for the UN Office for the Coordination of Humanitarian Affairs.

The UN has said it cannot deliver aid until "all guarantees are in place for safe delivery."

AFP`s correspondent in Aleppo`s eastern districts said the city was calm on Saturday after a few rocket attacks overnight.

According to the Syrian Observatory for Human Rights, two civilians were killed in regime shelling on the rebel-held town of Talbisseh, in central Homs province, on Saturday.

Since the truce came into effect, 47 civilians have been killed in areas controlled by the Islamic State group, including 38 killed in unidentified air strikes.

In all other areas across Syria, 12 civilians have been killed since the truce came into effect.

Under the US-Russia deal, if the truce lasts seven days and humanitarian access is granted, Moscow and Washington are to work together to target jihadists including the Islamic State group.