Russia on Wednesday called for a 48-hour extension of the Syria truce it brokered with the United States last week.
Moscow "calls for the extension of the cessation of hostilities on all Syrian territory for 48 hours," senior Russian military officer Viktor Poznikhir said in a briefing broadcast on state television.
He added that rebels had violated the ceasefire 60 times since it came into force on Monday.
"We also note that some groups controlled by the United States, such as Ahrar al-Sham, have openly stated their unwillingness to cease fire," Poznikhir said.
"The greatest number of violations is associated with these groups."
In a phone call with US Secretary of State John Kerry on Wednesday, Russian Foreign Minister Sergei Lavrov called on Washington to ensure it separates moderate rebels from militant groups.
Poznikhir also said that Russia had on Tuesday bombed the area north of the historic city of Palmyra, "where IS militants are concentrated," in the first such strikes since the truce came into force.
The strikes, Poznikhir said, had killed 250 people and up to 15 trucks containing heavy machine guns.
Poznikhir added that Moscow was "fully meeting its responsibilities" regarding the truce's implementation.
The truce that began at sundown on Monday, agreed after marathon US-Russia talks in Geneva between Lavrov and Kerry last Friday, is part of the latest bid to end a five-year conflict that has killed more than 300,000 people.
The US-Russian deal aims to end fighting between President Bashar al-Assad's loyalists and a wide range of rebels, but excludes militant forces like the Islamic State (IS) group.
The UN's Syria envoy Staffan de Mistura on Tuesday hailed the truce for bringing about "a significant drop in violence" despite isolated incidents.