Syrian rebels have violated the fragile ceasefire in the country 55 times over the past day, including with strikes on military and civilian targets in the divided city of Aleppo, according to Russia's military.

The Interfax news agency quoted Colonel Sergei Kopytsin as saying that mortar fire and improvised rockets struck Aleppo 26 times.

Russian news agencies cited another official as saying there had been 55 violations throughout the country.

The ceasefire brokered by Moscow and Washington came into effect on Monday. The truce has largely held, despite dozens of alleged violations by both sides.

The ceasefire was supposed to pave the way for aid shipments into besieged rebel-held areas of Aleppo, but convoys have yet to enter

The ceasefire was supposed to pave the way for aid shipments into besieged rebel-held areas of Aleppo, but convoys have yet to enter.

Meanwhile, Russian President Vladimir Putin has expressed frustration at Washington's refusal to publish details of the Syrian ceasefire deal reached with Russia, but said Moscow will not unilaterally release it.

"I don't really understand why we have to keep such an agreement closed," Mr Putin was quoted as saying during a trip to Kyrgyzstan.

He suggested Washington's resistance stems from a hope to retain the combat potential of forces fighting the Syrian government.

"This comes from the problems the US is facing on the Syrian track - they still cannot separate the so-called healthy part of the opposition from the half-criminal and terrorist elements," he said.

"In my opinion, this comes from the desire to keep the combat potential in fighting the legitimate government of Bashar Assad. But this is a very dangerous route."