Russian Foreign Minister Sergei Lavrov said on Wednesday that rebels in Syria's Eastern Ghouta were responsible for ensuring that a "humanitarian pause" in the area delivers relief for devastated civilians.
"Russia, together with the Syrian government, has already announced the establishment of humanitarian corridors in Eastern Ghouta," Mr Lavrov told the UN Human Rights Council.
"Now, it is the turn for the militants and their sponsors to act, militants entrenched there who still continue shelling Damascus, blocking aid deliveries and the evacuation of those wishing to leave," he said.
Tuesday was the first day of the five-hour daily "pause" in rebel-held Eastern Ghouta that was ordered by President Vladimir Putin, the Syrian regime's main ally.
But the truce was marred by violence, with Moscow and Damascus accusing armed groups of shelling the purported humanitarian corridor.
On Wednesday, Syrian regime forces continued to clash with rebels on the outskirts of the enclave.
The Britain-based Syrian Observatory for Human Rights said the fighting was fierce and broke out overnight in the rebel-held areas.
Early Wednesday, before the daily "humanitarian pause" took effect again, Syrian forces pounded Eastern Ghouta with air strikes and artillery fire, said the Observatory.
The Eastern Ghouta truce falls far short of a broader 30-day ceasefire Russia agreed to at the UN Security Council on Saturday but which has yet to take effect.
On Wednesday France's Foreign Ministry said that unlike rebel groups in the besieged area, the Syrian government has not committed to the UN-backed ceasefire.
Mr Lavrov said the Security Council resolution could offer reprieve to those suffering across Syria, but indicated the ball was in the court of the opposition and its allies in Washington.
"The people of Syria today face the most dire humanitarian crisis. UN Security Council resolution 2401 has established a framework for all parties to agree upon conditions to alleviate the plight of civilians throughout the territory of the country," he said.
"We call upon the members of the so-called American coalition to ensure the same humanitarian access to the areas in Syria under their control."
Also on Wednesday, Turkey sharply criticised France and the United States for arguing a ceasefire in Syria should apply to its military operation against the Kurdish militia People's Protection Units (PYD), as new tensions mounted between Ankara and its Nato allies.
Turkey has welcomed the UN's 30-day ceasefire but repeatedly insisted any truce would not affect its over month-long operation in the Afrin region against Kurdish militia Ankara believes to be terrorists.