By Michelle Nichols, Tom Perry and Humeyra Pamuk

UNITED NATIONS/DAMASCUS/ONCUPINAR, Turkey (Reuters) - World powers pressed Russia on Wednesday to stop bombing around Aleppo in support of a Syrian government offensive to recapture the city and a Western official said Moscow had presented a proposal envisaging a truce in three weeks' time.

Secretary of State John Kerry is pushing for a ceasefire and more aid access to Aleppo, where rebel-held areas are being cut off and the United Nations has warned a new humanitarian disaster could be on the way.

Aid workers said on Wednesday the water supply to Aleppo, still home to two million people, was no longer functioning.

Kerry is hoping for agreement at a meeting in Munich on Thursday between Russia, the United States, Saudi Arabia, Iran and other powers, aimed at trying to revive peace negotiations that foundered earlier this month.

Syrian officials have indicated no plans to ease up the war effort. A Syrian military source said on Wednesday the battle for Aleppo, a major prize in a war which has killed a quarter of a million people, would continue in "all directions".

Syrian Information Minister Omran al-Zoubi said the government expected a tough but relatively short battle to return the city to state control. "I do not expect the battle of Aleppo to go on long," he told Reuters in Damascus.

A Western official said Russia had made a proposal to begin a ceasefire in Syria on March 1, but that Washington has concerns about parts of it and no agreement had been reached.

In Washington, a state department envoy told Congress the United States needs to consider options in case the diplomatic push does not succeed.

Asked how soon a ceasefire could be put in place, a Russian diplomat who declined to be identified said: "Maybe March, I think so."

At a closed-door meeting of the 15-member U.N. Security Council on Wednesday, several members pressed Russia to end the Aleppo bombing sooner.

"The (Syrian) regime and its allies cannot pretend they are extending a hand to the opposition while with their other hand they are trying to destroy them," French U.N. Ambassador Francois Delattre told reporters.

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"CROSSED THE LINE"

Russian U.N. Ambassador Vitaly Churkin said Russian air strikes were being undertaken in a "transparent manner" and some Security Council members had "crossed the line" by politically exploiting humanitarian issues.

"They rather crudely use humanitarian matters in order to play, we believe, a destructive role as far as the political process is concerned," said Churkin, adding that given the heightened interest in humanitarian issues, the council should also start regularly discussing Yemen and Libya.

One U.N. diplomatic source said Russia was "stringing Kerry along" in order to provide diplomatic cover for Moscow's real goal - to help President Bashar al-Assad win on the battlefield instead of compromising at the negotiating table.

"It's clear to everyone now that Russia really doesn't want a negotiated solution but for Assad to win," said the diplomatic source, speaking on condition of anonymity.

The Kremlin rejects claims that it has abandoned diplomacy in pursuit of a military solution, saying it would continue to providing military aid to Assad to fight "terrorist groups" and accusing Syria's opposition of walking away from the talks.

FOOD, WATER SHORTAGES

Doctors working on both sides of the Syria-Turkey border say they have been overwhelmed by injuries caused by the air strikes, which Moscow says have only targeted Islamist militants but which Western countries say have caused widespread civilian casualties.

"We are increasingly seeing what we call multiple-trauma injuries because of the bombs and the heavy weapons they are using. There are large burn cases, lots of amputations, and internal traumas," Mahmoud Mustafa, director of the Independent Doctors Association, told Reuters in Gaziantep, Turkey.

French charity Medecins Sans Frontiers (MSF), which runs six hospitals in Syria and provides support for another 153 health facilities across the country, said medical workers in the area north of Aleppo had been forced to flee for their lives.

"Yet again we are seeing healthcare under siege," said Muskilda Zancada, MSF head of mission, Syria.

The International Committee of the Red Cross said it was delivering water to Aleppo because the city's system was no longer working but that some supply routes for aid had been cut.

"The temperatures are extremely low and, without an adequate supply of food, water and shelter, displaced people are trying to survive in very precarious conditions," the head of the ICRC in Syria, Marianne Gasser, said in a statement from Aleppo.

The latest fighting around Aleppo has killed about 500 people on all sides, a monitoring group said.

Medecins Sans Frontiers spokesman Sam Taylor said that while its own hospitals in Syria had not been hit, many others had.

"From the reports we get from MSF-supported facilities, the majority of hospitals are damaged or destroyed by aerial attacks," he said. "In last two to three weeks we have definitely seen a trend of facilities being hit in the south and in the north."

FABIUS QUESTIONS U.S. COMMITMENT

Saudi Arabia's King Salman plans to visit Moscow in mid-March, Russia's RIA news agency said, a meeting that would bring together the main sponsors of the opposing sides.

Saudi-backed rebels said they would go to Thursday's meeting in Munich but would only go to U.N. peace talks in Geneva later this month if Russia stopped bombarding their positions and humanitarian aid reached civilians in the areas they control.

Opposition coordinator Riad Hijab said the Russian and Iranian intervention in Syria was bolstering the extremist threat in the Middle East, but the rebels would not give up.

On the ground, rebels say they are fighting for survival.

A commander of a Turkmen contingent within the Levant Front rebel group, Zekeria Karsli, said his men faced attacks on three fronts: Islamic State to the east, Syrian government forces to the south and Kurds to the west.

"Unfortunately the military situation on the battlefield is pretty bad. Russian planes are hitting us from the air and the Iranian/Assad block is hitting us from the ground," he told Reuters near the Oncupinar border post.

He said Russian warplanes were carrying out hundreds of sorties every day and that the north of Aleppo city was encircled. But he said routes in to rebel-held parts of the city from Idlib province to the west were still open.

Opposition spokesman Salim al-Muslat said U.S. President Barack Obama could stop the Russian attacks. "If he is willing to save our children it is really the time now to say 'no' to these strikes in Syria."

The rebels want anti-aircraft weapons so they can bring down the Russian planes that have been bombing intensely over the past four months.

But their Western and Arab backers have refused, fearing Islamic State militants could seize and use them against their own planes conducting air strikes against the jihadists, who have exploited the war to seize large parts of Syria and Iraq.

United Nations Syria envoy Staffan de Mistura has set a target date of Feb. 25 to reconvene talks between the Syrian government and opposition in Geneva.

But the offensive by Syrian forces, Hezbollah and Shiite militias directed by Iran - all backed by Russian bombing raids - have reversed opposition gains on the ground and encircled rebels inside Aleppo, a strategic prize now divided between government and opposition control.

"It'll be easy to get a ceasefire soon because the opposition will all be dead," a Western diplomat told Reuters. "That's a very effective ceasefire."

(Additional reporting by Warren Strobel in Munich, John Irish in Paris, Louis Charbonneau in New York, Parisa Hafezi in Ankara, Tom Miles and Stephanie Nebehay in Geneva, Jonathan Landay in Washington and Michelle Nichols in New York; writing by Philippa Fletcher; editing by Dominic Evans)