The UN Security Council is struggling to agree a resolution seeking a ceasefire in Syria and has put back a vote until Saturday.
The vote has already been delayed several times since Thursday.
Russia wanted changes to a draft that calls for a 30-day calm to allow for aid deliveries and medical evacuations.
Western diplomats have accused Russia, Syria’s key ally, of stalling for time. France said failure to act may spell the end of the UN itself.
Minds have been focused amid outrage at the plight of civilians in the Eastern Ghouta rebel enclave.
Activists say 462 people have now been killed there this week.
Warplanes maintained the bombardment on Friday, witnesses said. Douma and Hamouriyeh were among areas hit.
Western powers suspect that Moscow wants to give Syria time to deal a final blow to forces defending the rebel-held enclave on the edge of Damascus.
The United States, the UK and France had called for the resolution to be approved without delay. UN special envoy Staffan de Mistura said a truce had to be followed by immediate, unhindered access to the Eastern Ghouta.
Commenting on Friday evening, US President Donald Trump pointed the finger of blame firmly at Syria and its allies, Russia and Iran, for the humanitarian crisis.
“What those three countries have done to people over the last, short period of time is a disgrace,” he said, speaking alongside the visiting Australian Prime Minister Malcolm Turnbull in Washington.
What does the draft resolution say?
The draft, put forward by Kuwait and Sweden, calls for a nationwide truce to go into effect 72 hours after the resolution is passed.
Medical evacuations and aid deliveries would start 48 hours after that. The draft says 5.6 million people in 1,244 communities across the country are in acute need.
Sweden’s UN ambassador Olof Skoog told the BBC that getting aid to the Eastern Ghouta, where conditions were described by the UN secretary general as “hell on earth”, was the main objective.
“I think that without the pressure coming from a united Security Council things are not happening the way they should on the ground,” he said.
“So I think for the council it’s a little bit less about the details and more about giving a political pressure to ensure that this happens.”
France’s UN ambassador François Delattre said the UN’s inability to help Syrian civilians would result in a devastating loss of credibility.
“The Syrian tragedy must not also become a graveyard for the United Nations,” he added.
The draft resolution also calls for all parties to avoid establishing military positions in civilian areas, including schools and hospitals. Sieges of populated areas should be lifted.
French President Emmanuel Macron and German Chancellor Angela Merkel wrote a joint letter to Russian President Vladimir Putin, urging him to back the resolution.
What are the Russian objections?
Under the terms of the draft resolution, any ceasefire would not apply to the Islamic State group, or the Nusra Front – formerly al-Qaeda’s official affiliate in Syria.
But Russian Foreign Minister Sergei Lavrov says it must go further and exclude other groups “co-operating with them” and which have shelled Damascus.
This could include the two biggest rebel groups in Eastern Ghouta – Jaish al-Islam and its rival Faylaq al-Rahman. Faylaq al-Rahman has in the past fought alongside the jihadist group Hayat Tahrir al-Sham, an alliance of factions led by the Nusra Front.
Mr Lavrov told reporters his country would be prepared to vote for the ceasefire under certain conditions.
“There are no guarantees that [the rebels] will not continue shooting at Damascus residential areas,” he told a briefing.
He said Russia had proposed “a formula which would make the ceasefire real”.
The sources said the stealth fighter planes were still undergoing flight tests and there was no confirmation they had yet been used in combat.
How bad is the situation in the Eastern Ghouta?
For the sixth day running, Syrian government forces have pounded the Eastern Ghouta.
A witness in Douma speaking to Reuters news agency early on Friday said the bombing there was the most intense so far.
Syrian state TV says rebels have shelled Damascus, killing one and injuring dozens more.
The US has also accused Russia of attacking Ghouta, allegations that Russian presidential spokesman Dmitry Peskov dismissed as “groundless”.
The number killed since Sunday in the Eastern Ghouta has risen to 462, the UK-based Syrian Observatory for Human Rights monitoring group says, at least 99 of them children.
Barrel bombs and shell fire have rained down on the area, where some 393,000 people remain trapped.
The Syrian government has denied targeting civilians and insisted it is trying to liberate the Eastern Ghouta from “terrorists” – a term it has used to describe both jihadist militants and the mainstream rebel groups that dominate the enclave.
Aid groups report dozens of hospitals being put out of action since Sunday.