* UN-Arab League envoy Annan pursues diplomatic effort

* Syria denies entry to U.N. humanitarian chief Amos (Adds UN humanitarian chief, Libyan aid pledge, background)

By Khaled Yacoub Oweis

AMMAN, Feb 29 (Reuters) - Heavy fighting raged near Baba Amro in Homs on Wednesday after elite Syrian troops attacked the rebel-held bastion that has endured 25 days of siege and fierce bombardment, activists said.

"Pray for the Free Syrian Army. Do not be miserly in your prayers for them," opponents of President Bashar al-Assad said in a statement, as diplomats spoke of his brother's feared 4th Armoured Division mounting a drive to "finish off" the rebels.

The motley band of army deserters and desperate insurgents who call themselves the Farouq Brigade of the Free Syrian Army have sworn to fight to the last man, one activist from Baba Amro told Reuters. Others, though, said some of the unit's leaders had already made their escape from the shattered neighbourhood.

The 4th Armoured Division commanded by Maher al-Assad, the president's younger brother, has a reputation for ruthlessness burnished during the past year of revolt and forged in history.

Drawn from the Alawite sect to which the Assads belong, it is hated by many in the Sunni majority who recall the role its predecessor units played in massacring many thousands of Sunni Islamists at Hama in 1982 on the orders of Assad's father Hafez.

Details from Homs were sketchy but as Syria refused to allow a visit to the country by a senior U.N. humanitarian envoy, Valerie Amos, a senior Western diplomat told Reuters: "All the signs out of Homs are that they're trying to finish it off.

"They clearly feel that letting her in now would be devastating for their image - as indeed it would be."

Communicating over the Internet, the Baba Amro activist, who calls himself only Ahmed and who said he had just left the area, said: "We call on all Syrians in other cities to move and do something to lift the pressure off Baba Amro and Homs.

"They should act quickly."

Homs, a symbol of opposition to Assad in a nearly year-long revolt, was without power or telephone links, Ahmed said.

Several Western journalists were trapped in the besieged district, although Syrian activists escorted British Sunday Times photographer Paul Conroy to safety in Lebanon on Tuesday in a messy escape in which some of his rescuers were killed.

CEASEFIRE PLEA

He had been among several journalists stranded in Baba Amro, where Marie Colvin, a veteran war correspondent also with the Sunday Times, and French photographer Remi Ochlik were killed in a bombardment on Feb. 22. Their bodies remain there.

It was not clear if Edith Bouvier, a wounded French freelance correspondent, Spanish journalist Javier Espinosa and French photographer William Daniels were still in Baba Amro.

YouTube footage posted by activists showed army trucks and tank carriers on a highway, purportedly heading for Homs.

Reports from the city could not immediately be verified due to tight government restrictions on media work in Syria, where Assad is facing the gravest challenge of his 11-year rule.

A spokesman for the International Committee of the Red Cross, Hicham Hassan, could not confirm the assault but said the violence was making the humanitarian situation more difficult.

"This makes it even more important for us to repeat our call for a halt in the fighting," he told Reuters in Geneva.

"It is essential that people who are in need of evacuation - wounded people, women and children - that we are able to offer them that with the Syrian Arab Red Crescent."

SHELLING, SNIPERS

Activists say hundreds of civilians have been killed in besieged opposition districts of Homs, including at least 20 on Tuesday. Shells and rockets have been crashing into Baba Amro since Feb. 4. Army snipers pick off civilians who venture out.

The ICRC said its Syrian Red Crescent affiliate had established 10 distribution and first aid points in Homs, but had been unable to operate in Baba Amro because of the violence.

Troops also bombarded the besieged town of Rastan, 20 km (13 miles) north of Homs, and several people were killed when a shell hit a house, the Syrian Observatory for Human Rights said.

Activists said troops and pro-Assad militiamen had also attacked the town of Helfaya, an opposition stronghold near Hama, detaining people and raiding and burning houses.

YouTube footage posted by activists showed crowds of people in the nearby town of Kernaz protesting in solidarity with Helfaya. Demonstrators danced, waved pre-Baathist era Syrian flags and chanted: "God support your oppressed subjects."

Troops and militiamen launched a security sweep in the eastern Damascus suburb of Harasta, where telephone services have been cut off for the past month, activists said.

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.

U.N. humanitarian chief Amos said she was "deeply disappointed" Syria refused to let her visit the country, where she had hoped to assess the emergency relief needs in besieged towns: "Given the rapidly deteriorating humanitarian situation, with an increasing need for medical assistance, food and basic supplies, improving access, so that assistance can reach those in urgent need, is a matter of the highest priority," she said.

DIPLOMATIC DRIVE

The senior Western diplomat who spoke of efforts to "finish off" the rebels in Homs said Syria had decided to deny Amos entry "despite Russian efforts to get her access".

The United States has outlined a new U.N. Security Council resolution on Syria, to demand access for relief workers and an end to violence, Western envoys said on Tuesday.

They said the draft focused on humanitarian problems to try to win Chinese and Russian support and isolate Assad, but that it would also suggest Assad was to blame for the crisis, a stance his longtime ally Russia has opposed.

Russia and China vetoed a draft resolution on Feb. 4 that would have backed an Arab League call for Assad to step down, but both nations have signalled support for humanitarian action.

Kofi Annan, the newly appointed U.N.-Arab League envoy for Syria, said he would hold talks in New York from Wednesday with U.N. Secretary-General Ban Ki-moon and member states. He will then meet Arab League chief Nabil Elaraby in Cairo.

Russian Deputy Foreign Minister Gennady Gatilov said that Annan, the former U.N. secretary-general, had also been invited to Moscow.

One obstacle to international efforts on Syria has been disunity among Assad's opponents, with no single group emerging as a credible and widely accepted representative.

Libya, one of the first states to recognise the opposition Syrian National Council as Syria's legitimate authority, pledged on Wednesday to give it $100 million in humanitarian aid, in another gesture of solidarity from a nation whose NATO-backed rebels overthrew Muammar Gaddafi. (Additional reporting by Dominic Evans, Erika Solomon and Mariam Karouny in Beirut, Steve Gutterman in Moscow, Louis Charbonneau at the United Nations, Taha Zargoun in Tripoli and Stephanie Nebehay in Geneva; Writing by Alistair Lyon; Editing by Alastair Macdonald)

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