"The Assad regime continues attacking its citizens, especially in East Ghouta," Pentagon spokeswoman Dana White said.

"We do not seek a conflict with the Syrian regime, but we call on Russia to restrain the Assad regime, de-conflict counterterrorism operations with the coalition, and de-escalate the remaining battlefields of the Syrian civil war."

Syrian regime forces have clashed with rebels on the outskirts of Eastern Ghouta, despite Russia on Monday ordering a daily "humanitarian pause" in the enclave to grant civilians safe passage to flee.

That move fell far short of a full ceasefire voted for by the United Nations Security Council.

"The failed ceasefire called into question Russia's commitment to de-escalate violence and negotiate a political solution," White said.

"Russia's inability to exert control over the portions of the battlefield where they operate is troublesome."

Despite the limited "humanitarian pause," the UN has insisted that all sides must implement the 30-day truce demanded on Saturday, and said some 40 trucks loaded with aid were waiting for the violence to stop.