BEIRUT - Most Syrian rebels pulled out of the besieged Baba Amro district of Homs on Thursday after a 26-day siege by Syrian President Bashar 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.'They won't take it, 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 organize 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 defense ministry."The SNC has been criticized by some Syrians for not overtly backing the armed struggle led by the loosely organized 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 UN-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 UN 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 UN 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 UN 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.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 UN Human Rights Council which condemned Syria for violations that may amount to crimes against humanity.