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.
The Farouq Brigade of the Free Syrian Army was trying to hold off the assault led by units of the armoured Fourth Division, which is commanded by Maher al-Assad, the hardline brother of President Bashar al-Assad.
Pray for the Free Syrian Army. Do not be miserly in your prayers for them, the activists said in a statement.
The rebels have sworn to fight to the last man, according to Ahmed, an activist who said he had just left Baba Amro. He said other opposition areas of Homs were also under attack but gave no details of casualties.
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, Ahmed said, contacted via Skype.
However some activists said leaders of the Farouq Brigade had already left Baba Amro.
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 Baba Amro district, although Syrian activists escorted British Sunday Times photographer Paul Conroy to safety in nearby Lebanon on Tuesday in a messy escape in which some of his rescuers were killed.
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 February 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.
The U.N. humanitarian chief Valerie Amos said Syria had refused to allow her to visit the country, where she had hoped to assess the need for emergency relief in conflict areas.
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 February 4. Army snipers pick off civilians who venture out.
Syrian troops 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 the city of Hama, detaining people and raiding and burning houses.
Youtube footage posted by activists showed crowds of people in the nearby town of Kernaz 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.
As world dismay grew over the bloodshed, France said the Security Council was working on a new Syria resolution and urged Russia and China not to veto it, as they have previous drafts.
An outline drafted by Washington focused on humanitarian problems to try to win Chinese and Russian support and isolate Assad, Western envoys said. But they said the draft would also suggest Assad was to blame for the crisis, a stance his longtime ally Russia in particular has opposed.
Russia and China vetoed a draft resolution on February 4 that would have backed an Arab League call for Assad to step down. China indicated a possible shift late on Tuesday when it told the head of the Arab League it supported international efforts to send humanitarian aid to Syria.
But Chinese Foreign Minister Yang Jiechi also urged political dialogue in Syria, something ruled out by Assad's opponents while the bloodshed goes on, and Russia has warned against interference in Syria under a humanitarian guise.
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 the former U.N. secretary-general had also been invited to Moscow.
(Additional reporting by Dominic Evans, Erika Solomon and Mariam Karouny in Beirut, Steve Gutterman in Moscow, Louis Charbonneau and Michelle Nichols at the United Nations and Stephanie Nebehay in Geneva; Writing by Alistair Lyon; Editing by Rosalind Russell)