(CNN) Beleaguered civilians in Syria's rebel-held Eastern Ghouta say they are being assailed by a new kind of rocket that spreads potentially deadly fires, as human rights groups condemned international inaction over the crisis.

The Syrian American Medical Society told CNN that 100 of the rockets had rained down in recent days, and that civil defense forces were struggling to put out the fires they caused.

Close to 400,000 people are living in deteriorating conditions in the Damascus suburb, which has been pounded with shells, mortars and bombs by Russian-backed Syrian regime forces since Sunday night.

More than 400 civilians, including women, children and the elderly, have been killed since Sunday, the head of Eastern Ghouta's health department, Dr Fayez Orabi, told CNN on Friday. More than 2,000 others are injured, a quarter of them severely, he said.

"It's difficult to have a precise count because of the internet and communications are weak and the shelling and bombing are 24 hours," Orabi said via WhatsApp, adding that rockets were continuing to fall as he wrote.

The UN Security Council failed to vote Thursday on a draft resolution which called for a 30-day halt in the fighting to allow for critical aid deliveries and medical evacuations. The United States accused Russia -- the Syrian regime's main ally -- of blocking the measure. The Security Council met again on Friday to consider the temporary ceasefire, but ultimately postponed the vote until Saturday.

Russian Foreign Minister Sergey Lavrov said Friday that Russia is ready to vote for a ceasefire resolution but that the United States and its allies won't provide guarantees that militants in Syria will observe it, according to Russia's state-run RT outlet.

"For now, they refuse to accept an amendment which will place responsibility on them to ensure that the militants give clear guarantees to stop the shelling," Lavrov was quoted as saying.

Human Rights Watch called for immediate action. "Other countries should send a clear message to Syria's chief enabler, Russia, that it needs to end its efforts to block the Security Council from taking action to stop these atrocities," said Lama Fakih, the campaign group's deputy Middle East director.

'Widespread fires'

Basel Termanini, vice president of the Syrian American Medical Society, told CNN that civilians in Eastern Ghouta, many of whom are sheltering from the bombardment in makeshift underground shelters, were now under threat from fires.

"People from inside Ghouta are reporting new rockets that are causing widespread fires which is a new development," Termanini said. "More than 100 of those were launched today and the civil defense is unable to cope with the widespread fires. Fire is now the number one danger threatening civilians."

Photos: Airstrikes in Eastern Ghouta Medics tend to a baby as a child cries next to them at a makeshift clinic in Douma, Syria, on Thursday, February 23. More than 400 civilians have been killed this week in Syria's rebel-held Eastern Ghouta region, according to the head of the region's health department on Friday, February 23. Syrian regime forces have been pounding Eastern Ghouta with shells, mortars and bombs. Hide Caption 1 of 15 Photos: Airstrikes in Eastern Ghouta A man rescues a child after a reported airstrike in the rebel-held town of Hamouria on Wednesday, February 21. Syria says it is targeting terrorists in Eastern Ghouta. Hide Caption 2 of 15 Photos: Airstrikes in Eastern Ghouta Members of a Syrian civil-defense team rescue a man in Hamouria on February 21. Hide Caption 3 of 15 Photos: Airstrikes in Eastern Ghouta A woman and children run for cover after bombing in Hamouria on Monday, February 19. Hide Caption 4 of 15 Photos: Airstrikes in Eastern Ghouta Hide Caption 5 of 15 Photos: Airstrikes in Eastern Ghouta An injured child cries as he receives treatment at a makeshift hospital in Hamouria on February 19. Hide Caption 6 of 15 Photos: Airstrikes in Eastern Ghouta A man carries an injured victim amid the rubble of buildings. Hide Caption 7 of 15 Photos: Airstrikes in Eastern Ghouta A boy waits to receive medical treatment at a field hospital on February 19. Hide Caption 8 of 15 Photos: Airstrikes in Eastern Ghouta Injured children are treated at a hospital in Douma. Hide Caption 9 of 15 Photos: Airstrikes in Eastern Ghouta A wounded boy receives treatment in Douma following airstrikes on the village of Mesraba. Hide Caption 10 of 15 Photos: Airstrikes in Eastern Ghouta A man carries an infant he rescued from the rubble in Hamouria on February 19. Hide Caption 11 of 15 Photos: Airstrikes in Eastern Ghouta Injured children are treated at a hospital in Douma. Hide Caption 12 of 15 Photos: Airstrikes in Eastern Ghouta Ghaith, a wounded 12-year-old boy, cries as he waits for treatment -- and news of his mother in the operating room -- at a makeshift hospital in Kafr Batna. They were hurt in airstrikes on the town of Jisreen. Hide Caption 13 of 15 Photos: Airstrikes in Eastern Ghouta Children cry at a makeshift hospital in Douma. Hide Caption 14 of 15 Photos: Airstrikes in Eastern Ghouta A man weeps over his child, who was killed in the Mesraba airstrikes. Hide Caption 15 of 15

Dr. Hamza Hassan, a surgeon working in Eastern Ghouta, told CNN via WhatsApp voice messages that there had been an extremely heavy bombardment Thursday afternoon and evening.

He described seeing "Syrian regime planes, Russian planes and helicopters" overhead, with so many at a time that it was like watching a military parade.

"They were bombing us from the sky, land and everywhere. They are using new planes, new sounds to us, we never heard before," he said. "The offensive was so intense and artillery shelling hitting the areas with intense bombing and rockets and missiles from all over and burning everything."

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Some of the weapons used "burned like we have never seen before," Hassan said.

After a yearslong siege, food, water and drugs are in desperately short supply in Eastern Ghouta and injured civilians have little recourse to help. The Union of Medical Care and Relief Organizations said 26 medical facilities had been targeted by strikes between Monday and Thursday.

Syria says it is targeting terrorists in Eastern Ghouta. Rebel groups in the area have fired mortars into Damascus this week, causing dozens of deaths and injuries, state media report.

Russia has sought to lay the blame for the crisis at the door of the rebel groups, saying they have derailed talks to resolve the conflict and are preventing civilians from leaving the enclave.

Syrians look for survivors at the site of strikes in Douma, in Eastern Ghouta, on Thursday.

Lavrov: No guarantee militants will observe ceasefire

French President Emmanuel Macron and German Chancellor Angela Merkel sent a letter Friday to Russian President Vladimir Putin asking for a ceasefire in Eastern Ghouta, the Elysee Palace said.

EU foreign policy chief Federica Mogherini issued a statement insisting it was the "responsibility of all" to end the violence. "The Syrian regime must immediately stop targeting its own people and fulfill its primary responsibility to protect them," she said.

The European Union backs the UN call for a ceasefire and expects the rest of the international community to do likewise, Mogherini added.

Inujured Syrian children cry as they wait to receive medical treatment at a field hospital after strikes on Kafr Batna in Eastern Ghouta on Thursday.

Pressed Thursday to describe what the US was doing to end the violence, US State Department spokeswoman Heather Nauert expressed frustration with the media. "I don't know what some of you expect us to do," she exclaimed. Nauert insisted that the administration was "fully engaged" with the crisis.

Russia's Ambassador to the United Nations Vassily Nebenzia, who argued against the UN draft resolution, claimed that there was a "mass psychosis by global media" over the situation in Eastern Ghouta.

Regime drops leaflets

The Syrian military said Syrian army helicopters dropped leaflets on Eastern Ghouta, calling for residents to leave the area for their safety. A photograph of the leaflets was published on the Syrian military's social media accounts.

The leaflets provide instructions on how to get safely out of Eastern Ghouta and warn that the area is surrounded by the Syrian army. The text calls for civilians "not to deal with the militants."

Hassan told CNN on Friday that he had seen images of the leaflets that were released by the Syrian military online, but that no one he knew had seen the actual fliers.

A medical aid worker who spoke to CNN on Thursday evening described how people were staying in packed basements day and night for fear of the bombing.

"I just surfaced from the shelter basement of a building, I haven't been outside the shelter all day because the intensity of the bombardment and airstrikes," he said. "I just surfaced to use the internet and check the news. People are crammed all over each other in the basements. No one is in the street, it's dark now and still we hear the jets above." He then retreated back below ground.

Syrians prepare to bury a body in Kafr Batna, in the besieged Eastern Ghouta region, on Thursday.

The Syrian military leaflets, which blame insurgents for the deaths of thousands of women and children, promise that residents who leave will receive food, shelter and medical assistance in addition to a safe return home "once terrorism is eliminated."

The leaflets are similar to those dropped over rebel-held eastern neighborhoods of the northern city of Aleppo, before the besieged area was taken by government forces in December 2016.

This week's intense bombardment has prompted fears that a ground offensive could soon be launched against Eastern Ghouta.