Most Syrian rebels pulled out of the besieged Baba Amro district of Homs on Thursday after a 26-day siege by President Bashar al-Assad's forces, activists in contact with the fighters said.

They said a few fighters had remained behind in the shattered quarter to cover the 'tactical withdrawal' of their comrades.

Syrian forces again shelled Baba Amro earlier in the day, despite world alarm at the plight of civilians trapped there.

Snow blanketed the city, slowing a ground assault begun on Wednesday, but also worsening the misery of residents short of food, fuel, power, water and telephone links, activists said.

Reports from the city could not be verified immediately due to tight government restrictions on media operations in Syria.

Assad is increasingly isolated in his struggle to crush an armed insurrection that now spearheads a year-long popular revolt against four decades of his family's iron-fisted rule.

But he still has some allies. Russia, China and Cuba voted against a resolution adopted overwhelmingly by the Geneva-based U.N. Human Rights Council which condemned Syria for violations that may amount to crimes against humanity.

'WHATEVER THE COST'

A Lebanese official close to Damascus said Assad's government was determined to regain control of Homs, Syria's third city, which straddles the main north-south highway.

"They want to take it, whatever happens, without restraint, whatever the cost," the official said, asking not to be named.

He said defeat for the rebels in Homs would leave the opposition without any major stronghold in Syria, easing the crisis for Assad, who remained confident he could survive.

The exile opposition Syrian National Council said it had formed a military bureau to oversee and organise armed anti-Assad groups under a unified leadership.

"The creation of the military bureau was agreed upon by all armed forces in Syria," SNC leader Burhan Ghalioun told a news conference in Paris. "We will be like a defence ministry."

The SNC has been criticised by some Syrians for not overtly backing the armed struggle led by the loosely organised Free Syrian Army, made up of army deserters and other insurgents.

There was no immediate comment from the rebel army.

With Assad's forces closing in on rebels in Homs, the SNC appealed for help late on Wednesday, urging the U.N.-Arab League envoy on Syria, Kofi Annan, to go to Baba Amro "tonight".

Annan said in New York he expected to visit Syria soon and urged Assad to engage with efforts to end the turmoil.

Syria, which denied entry this week to U.N. humanitarian aid chief Valerie Amos, adopted a guarded approach to Annan's role.

The state news agency SANA quoted Foreign Ministry spokesman Jihad al-Maqdisi as saying the government was 'waiting for a clarification from the U.N. on the nature of his mission'.

The ministry also said it was ready to discuss a date for Amos to visit instead of the 'inconvenient' one she had sought.

Russian Deputy Foreign Minister Gennady Gatilov said in Geneva he hoped Syria would let Amos in soon.

Russia, which along with China, has shielded Syria from U.N. Security Council action, is emerging as a pivotal player in diplomacy aimed at halting the bloodshed and relieving the humanitarian crisis facing civilians caught in conflict zones.

Moscow has invited Annan for talks on Syria and, according to Kuwaiti officials, will send Foreign Minister Sergei Lavrov to meet his Gulf Arab counterparts in Riyadh next week.

ARMING REBELS

Saudi Arabia and Qatar have led calls for the world to arm Syrian rebels following last month's Russian-Chinese veto of a draft U.N. Security Council resolution on Syria.

Syria's Maqdisi told Lebanon's Hezbollah-run al-Manar television that the Saudis and Qataris were "singing from the same hymn sheet" as al Qaeda leader Ayman al-Zawahri, who has urged Arabs and Muslims to support anti-Assad insurgents.

Kuwait's parliament, dominated by Sunni Islamists, said it had agreed to support the Free Syrian Army and urged the Kuwaiti government to cut relations with Syria.

While the Sunni Gulf monarchies have been alarmed by demands for democracy inspired by popular revolts across the Arab world, they have also long been at odds with Shi'ite Iran, their non-Arab rival on the other side of the Gulf, and with Tehran's Arab allies, Alawite-ruled Syria and the Shi'ites of Hezbollah.

The grievances of Syria's Sunni majority under Assad, whose Alawite sect is an offshoot of Shi'ite Islam, have struck a chord with both the public and wealthy rulers in the Gulf.

The United States and its allies are seeking a new Security Council resolution on Syria, which Western envoys said would focus on humanitarian problems to try to win Chinese and Russian support, but would also criticise Assad.

Wednesday's army ground assault on Baba Amro followed more than three weeks in which Assad's forces have bombarded rebel enclaves in Homs with rockets, shells and mortar rounds.

"Homs is cut off from the world," an activist statement said. "Martyrs are being buried in gardens and parks because the presence of army snipers prevents taking them to cemeteries."

It said hospitals were only treating pro-Assad militiamen, while makeshift medical centres had run out of medicine.

Western and Arab governments, which have already called on Assad to step down and end the bloodshed, expressed mounting concern for civilians struggling to survive in Homs.

"I am appalled by reports that the Assad regime is preparing a full-scale land assault on the people of Homs," Britain's Foreign Secretary William Hague said.

Britain has withdrawn all diplomatic staff from Syria and suspended services at its embassy in Damascus in response to worsening security in the country, a diplomat said on Thursday.

The United Nations says Assad's security forces have killed more than 7,500 civilians since the revolt began last March. Syria's government said in December that "armed terrorists" had killed more than 2,000 soldiers and police during the unrest.