The White House said Russian military aircraft took off from Humaymim Airfield in Syria and carried out at least 20 daily bombing missions in Damascus and eastern Ghouta between February 24 and February 28.

It did not say whether the jets dropped ordnance, which could be harder to determine than tracking the flight paths of Russian aircraft on US radar. But the United States directly accused Russia of killing civilians.

"Russia has gone on to ignore (a UN ceasefire's) terms and to kill innocent civilians under the false auspices of counter-terrorism operations," the White House said in a statement.

Syrian President Bashar al-Assad vowed on Sunday to continue the offensive in eastern Ghouta, one of the deadliest in the war. A local insurgent group called it a "scorched earth" campaign.

With the war entering its eighth year, capturing eastern Ghouta would be a major victory for Assad, who has steadily regained control of rebel areas with Russian and Iranian support.

Government shelling and air strikes have killed 659 people in eastern Ghouta since February 18, while rebel shelling of Damascus has killed 27, according to the Syrian Observatory for Human Rights.

The United Nations High Commissioner for Human Rights said on Friday that Syrian government air strikes on eastern Ghouta and shelling from the rebel-held zone into Damascus probably constitute war crimes.

The White House called on pro-Assad forces to "immediately cease targeting medical infrastructure and civilians."

In a separate statement later on Sunday, the White House said President Donald Trump and Egypt's President Abdel Fattah al-Sisi "discussed Russia and Iran's irresponsible support of the Assad regime's brutal attacks against innocent civilians."

"President Trump and President al-Sisi agreed to work together on ending the humanitarian crisis in Syria and achieving Arab unity and security in the region," the White House statement said.

Russia and Damascus have accused rebels of preventing civilians from leaving eastern Ghouta during daily ceasefires. Rebels have consistently denied the accusation and say people will not leave because they fear the government.

The multi-sided war has killed hundreds of thousands of people since 2011.