Syrian forces overran the Baba Amr district of powderkeg Homs after rebels retreated, potentially marking a turning point in President Bashar al-Assad's bid to crush an increasingly armed uprising.
As rebel fighters pulled back, the opposition Syrian National Council (SNC) warned of a "massacre" in the rebel neighbourhood by Syrian forces, while relief agencies said they would urgently try to get there to deliver aid and evacuate the wounded.
Later on Thursday, the UN Security Council called on Syria to allow "immediate" humanitarian access to protest cities in a statement also supported by Russia and China, who had previously vetoed two resolutions.
The SNC, citing "confusion" on the ground in Syria, said in Paris it would provide leadership to an outgunned and fragmented force and control the flow of arms to fighters.
The rebels said they had pulled out "tactically" from Baba Amr Thursday, the second day of an all-out ground assault by the feared Fourth Armoured Division, led by a younger brother of President Bashar al-Assad, Maher.
The storming of the rebel bastion which began early on Wednesday and continued into Thursday, following 27 straight days of relentless shelling of Homs by regime forces.
Rebels "have pulled out tactically in order to protect the remaining civilians," said Colonel Riyadh al-Asaad, the leader of the Free Syrian Army, which is made up mostly of deserters.
The FSA was formed mid-2011 in response to a brutal crackdown by Assad's forces on anti-regime protesters, and now boasts up to 40,000 armed fighters, although the numbers are impossible to verify.
A Syrian security official said in Damascus that the army was in total command of the Homs neighbourhood, which had become the symbol of resistance to the regime.
"The Syrian army controls all of Baba Amr. The last pockets of resistance have fallen," the official told AFP.
State television aired footage it said was filmed inside Baba Amr, including interviews with people it said were residents angry with the rebels.
The SNC urged the international community to act to prevent to protect residents, charging that the Fourth Armoured Brigade was conducting "barbaric operations against civilians."
"We urge the international community, Muslim and Arab states to intervene immediately to prevent a potential massacre in the coming hours against tens of thousands of children, women and elderly people," it said.
The Syrian Observatory for Human Rights said 21 people were killed in Homs on Thursday, including 17 civilians caught up in the battle for control of Baba Amr.
Global campaigning organisation, Avaaz, said the 17 were "beheaded or partially beheaded" in the farming area on the outskirts of Baba Amr.
In total, 39 people, including eight loyal soldiers and seven deserters, were killed in violence across Syria on Thursday, the Observatory said.
The International Committee of the Red Cross and the Syrian Arab Red Crescent were preparing to urgently reach the conflict zone, an ICRC spokesman said.
"The ICRC and the SARC will go on Friday to Baba Amr to deliver humanitarian aid and evacuate the wounded," Damascus spokesman Saleh Dabbakeh told AFP.
French journalist Edith Bouvier, who suffers from multiple fractures, is believed still trapped inside Baba Amr.
On the political front, the SNC said its military bureau, announced on Wednesday, would coordinate the flow of weapons to the rebels following mounting calls from Gulf Arab states for arms to be delivered despite US fears that Al-Qaeda may exploit any further militarisation of the crisis.
"The SNC will be this link between those who want to help and the revolutionaries," its leader Burhan Ghalioun told reporters in Paris.
"It is out of the question that arms go into Syria in confusion," he added.
The all-out assault on Homs's defiant neighbourhood came as international envoy Kofi Annan said he hopes to go to Damascus with a clear message that the "violence must stop."
Britain announced that it was following the United States in closing its embassy and pulling out its remaining diplomats in response to the "deterioration of the security situation in Damascus."
UN political chief B Lynn Pascoe told the Security Council on Tuesday that "well over 7,500" people have been killed since Assad's forces began a crackdown on anti-regime protests that erupted in March last year.
In New York, the UN Security Council called on Syria to allow "immediate" humanitarian access to protest cities.
Russia and China significantly signed up to the statement after Syria refused to let UN humanitarian chief Valerie Amos into the country. Syria has said the date proposed by Amos was not suitable.
"The members of the Security Council express their deep disappointment" that Amos "was not granted authorisation to visit Syria by the Syrian government in a timely manner, despite repeated requests and intense diplomatic contacts aimed at securing Syrian approval," said the statement.
The 15-nation body called for "immediate and unhindered access" for Amos.