Russia and Syrian rebels have cast doubt over the prospects for an increasingly shaky five-day-old ceasefire, with Moscow saying the situation is worsening and a senior insurgent warning the truce "will not hold".
The ceasefire is the result of an agreement between Russia, which backs Syrian President Bashar al-Assad with air power, and the United States, which supports some rebel groups, and has cooled fighting since coming into effect on Monday.
However some violence has persisted across Syria and promised aid deliveries to besieged areas remain blocked with both sides accusing the other of bad faith.
Russia's Defence Ministry said on Saturday conditions were deteriorating, adding that it believed the ceasefire had been breached 199 times by rebels and saying the United States would be responsible if it were to collapse.
Earlier on Saturday, Russian President Vladimir Putin cast doubt over Washington's commitment but also said he believed securing a ceasefire was a common goal for the two countries, which both agreed to extend on Friday.
Insurgents say they only reluctantly accepted the initial deal, which they believe is skewed against them, because it could relieve the dire humanitarian situation in besieged areas they control. They blamed Russia for undermining the truce.
"The truce, as we have ... told the (US) State Department, will not hold out," the rebel official said, pointing to the continued presence of a UN aid convoy at the Turkish border awaiting permission to travel to Aleppo.
"It is not possible for the party (Russia) that wages war against a people to strive to achieve a truce, as it is also not possible for it to be a sponsor of this agreement while it bombs night and day, while on the other side, the other party - America - has the role of spectator," he said.
Moscow has itself accused rebels of breaking the truce and says Washington needs to do more to make them abide by its terms, including separating from the jihadist Jabhat Fateh al-Sham, which only broke formal allegiance to al Qaeda in July.
The five-year-old civil war has killed hundreds of thousands and displaced half the country's population, drawing in global and regional powers, causing an international refugee crisis and inspiring jihadist attacks around the world.
Both sides have accused the other of being responsible for aid deliveries being stuck far from Aleppo, where army and rebel forces were supposed to pull back from the Castello Road which leads into besieged, insurgent-held eastern districts.
Russia on Friday said the Syrian army had initially withdrawn but returned to its positions after being fired on by rebels, who in turn say they saw no sign of government forces ever leaving their positions.
If the ceasefire deal is successful, Moscow and Washington will start to share targeting information on militant groups, including Islamic State, they have said.