(Updates death toll)
* U.N. envoy urges Security Council to act to stop "massacre"
* Security Council considering resolution for 30-day truce
* U.S. seeks quick vote on resolution, Russia wants changes
* At least 416 people killed in five days of aerial bombing
* Assad bent on retaking last rebel enclave near Damascus
By Suleiman Al-Khalidi
AMMAN, Feb 22 (Reuters) - Warplanes pounded the last rebel enclave near Syria's capital for a fifth day running on Thursday as the U.N. Security Council considered demanding a 30-day ceasefire across the country to allow emergency aid deliveries and medical evacuations.
The U.N. envoy for Syria, Staffan de Mistura, pleaded for a truce to halt one of the fiercest air assaults of the seven-year civil war and prevent a "massacre" in the besieged eastern Ghouta region on the outskirts of Damascus.
At least 416 people have been killed in eastern Ghouta since Sunday night, according to the Syrian Observatory for Human Rights war monitor, with more than 2,100 wounded from the assault by Syria's military and its allies.
Planes have struck residential areas in the enclave of 400,000 people and, said medical charities, hit more than a dozen hospitals, making it near impossible to treat the wounded.
Panos Moumtzis, the U.N. humanitarian coordinator for Syria, said households in eastern Ghouta were without food, water or electricity in winter cold and 80 percent of the population of the town of Harasta was living underground.
"There is a need for avoiding a massacre, because we will be judged by history," Mistura said, urging the U.N. Security Council to act. The Council was meeting on Thursday to discussion the situation at the request of Russia.
President Bashar al-Assad's main ally Russia, which wields a veto on the Security Council, said it could support a 30-day truce, but not one that included the Islamist militants it says the onslaught on eastern Ghouta is meant to target.
Aid workers and residents say Syrian army helicopters have been dropping "barrel bombs" - oil drums packed with explosives and shrapnel - on marketplaces and medical centres.
Residents and insurgents in eastern Ghouta say Russian planes are also involved. Syrians say they can identify Russian aircraft because they fly at higher altitude than Syrian planes.
Damascus and Moscow deny using barrel bombs or hitting civilians. They say rebels hold civilians as human shields.
The Security Council was considering a resolution, drafted by Kuwait and Sweden, that demands "a cessation of hostilities throughout Syria for all military operations except those directed at the Islamic State ... al Qaeda and al Nusra Front" for 30 days to allow aid deliveries and medical evacuations.
Swedish U.N. Ambassador Olof Skoog said he hoped the Council could vote on the resolution on Thursday. But Russian Ambassador Vassily Nebenzia said he would propose amendments to the text for "it to be realistic."
Deputy U.S. Ambassador to the United Nations Kelley Currie accused Russia of "appearing to be intent on blocking any meaningful effort" to halt the bloodshed in eastern Ghouta.
"What we need is a sustained cessation of hostilities and we need it desperately," U.N. aid chief Mark Lowcock told the gathering. "Millions of battered and beleaguered children, women and men depend on meaningful action by this Council."
Previous attempts at a cessation of hostilities in Syria have quickly unraveled.
Residents of Douma, the biggest town in eastern Ghouta, described plumes of black smoke billowing from residential areas after planes dropped bombs from high altitude.
Searches were under way for bodies amid the rubble in the town of Saqba and elsewhere, said rescuers.
Syrian army helicopters dropped fliers over eastern Ghouta, according to a media unit run by Assad's Lebanese Hezbollah ally.
The fliers called on civilians to hand themselves over to the Syrian army in order to save their lives, with a passage highlighted on a map for a safe journey out of eastern Ghouta.
Sara Kayyali, Syria researcher for Human Rights Watch, said the situation in eastern Ghouta was deteriorating "with over 250 civilians dead in the last 48 hours. "Witnesses that we are speaking to on the ground are saying that it's 'raining bombs'," she told Reuters in Geneva.
Robert Mardini, Middle East regional director for the International Committee of the Red Cross, said the ICRC was poised to offer emergency medical care in the enclave and carry out evacuations of wounded as soon as conditions permitted.
In Syria's north, where Turkey launched an offensive in the past month against a Kurdish militia, the Kurds said pro-government fighters were now deploying to front lines to help repel the Turkish advance, though assistance would be needed from the Syrian army itself.
The Kurdish YPG - backed by the United States in other parts of Syria - have sought help recently from the Russian-backed Damascus government to resist the Turkish thrust - an example of the strange bedfellows in a multi-sided conflict that has drawn in neighbours and world powers.
International attention is now focused on the humanitarian emergency in eastern Ghouta, whose population has been under siege for years and where government bombardments escalated sharply on Sunday, causing mass civilian casualties.
Moscow and Damascus say their assault on eastern Ghouta is necessary to defeat rebels who have been firing mortars on the capital - government territory throughout the war.
"Those who support the terrorists are responsible" for the situation in eastern Ghouta, Kremlin spokesman Dmitry Peskov told a conference call with reporters. "Neither Russia, nor Syria nor Iran are in that category of states, as they are waging an absolute war against terrorists in Syria."
Video footage obtained by Reuters showed wreckage at the Al Shifa hospital in the town of Hammouriyeh. Staff said it had been hit by air strikes and artillery.
"The clinics department is out of service, the clinical care unit is out, the surgery unit is out, the incubator unit is out, the pediatric section is out, all of the departments of the hospital are completely out of service," a man identified as a medical worker said.
Eastern Ghouta has been under siege by the Syrian army and allied forces since 2013. After government gains, it is the final rebel bastion near the capital.
Along with Idlib province in the north, part of Aleppo province and a strip in Syria's southwest, it is one of a handful of areas left where large numbers of people remain in territory controlled by fighters seeking to overthrow Assad. The president has vowed to regain control of every inch of Syria.
(Reporting by Suleiman Al-Khalidi; Additional reporting by Ellen Francis, Lisa Barrington, Dahlia Nehme and Angus McDowall in Beirut, Tuvan Gumrukcu in Ankara, Stephanie Nebehay and Tom Miles in Geneva, Polina Nikolskaya in Moscow and Michelle Nichols at the United Nations; Editing by Yara Bayoumy and Alistair Bell)
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