Dozens were killed in the Syrian city of Homs on Thursday, activists said, in a burst of violence following reports of sectarian killings this week that threaten to ignite civil strife between majority Sunnis and the minority Alawite sect.

Tanks pounded a main residential district in Homs for the second day, and there was no sign of troops leaving cities under an Arab League agreement to end bloodshed after seven months of protests against President Bashar al-Assad, whose Alawite sect dominates power.

A witness, who declined to be named, said he saw dozens of civilian bodies at the National Hospital, which is under security forces' control. The circumstances of their death were not clear.

They were all males with bullet wounds. A doctor told me they came from all over Homs, he said.

Activists said a further 19 people were killed in tank shelling of the Bab Amro district, a hotbed of pro-democracy protests against President Bashar al-Assad, and in shooting by security forces elsewhere in Homs.

There was no independent confirmation of the killings. They follow reports by local activists that forces loyal to Assad shot dead at least 11 Sunni Muslim villagers they had stopped at a roadblock northwest of Homs on Wednesday.

Nine Alawites had been dragged from a bus and killed near the city the day before, another activist said.

Tough Syrian media restrictions have made it hard to verify events on the ground since an uprising against Assad began in March, inspired by other revolts in the Arab world.

Syria, under mounting pressure to halt a crackdown that the United Nations says has killed over 3,000 people, agreed on Wednesday to an Arab League plan to pull the army out of cities, free political prisoners and hold talks with the opposition.

The authorities blame the violence on Islamist militants and armed gangs who they say have killed 1,100 soldiers and police.

ISOLATION FEAR

Arab League Secretary-General Nabil Elaraby briefed members of the main opposition National Council on the plan in Cairo.

We did not talk with the secretary-general about a dialogue with the regime, council member Samir Nashar was quoted by Egypt's MENA news agency as saying after the meeting.

We discussed entering negotiations with the authorities to move from a totalitarian to a democratic system, and demanded that President Assad leaves power.

Assad's critics have dismissed his past offers of dialogue, with some saying bloodshed must stop first and others saying that only his resignation can end the conflict.

Paris-based Burhan Ghalioun, a leading National Council member, questioned whether Syria would implement the plan.

The regime has accepted the Arab initiative out of fear of Arab isolation, its weakness and lack of options. But its acceptance does not mean it will respect its clauses, he wrote.

In Syria, some demonstrators celebrated the Arab League initiative, but residents and activists said troops remained in cities and security operations continued, especially in Homs.

Tanks fired heavy machineguns and anti-aircraft guns in the city's Bab Amro district, a hotbed of protests and scene of operations by the military against insurgents hiding there.

In other districts of Homs, a city of one million, army snipers were shooting from rooftops and soldiers fired from checkpoints.

We slept late because there were overnight street rallies celebrating the Arab initiative. This morning we woke up to rain and shelling, Samer, an activist in Bab Amro, said by phone.

EXPLOSIONS ROCK CITY

Another resident said the sound of explosions was rocking the city, and activists were calling on people to donate blood for makeshift hospitals in and around the centre of Homs.

Activists and residents reported army reinforcements at roadblocks in towns across the southern Hauran Plain, where troops fired in the air to disperse overnight protests.

Early in the morning, an armoured column fired machineguns in the air after entering al-Madiq castle near the Roman ruins of Apamea in the Ghab Plain, which has seen protests and has emerged as a refuge for army defectors, local activists said.

In the Damascus suburb of Harasta, at least 120 protesters were arrested overnight after celebrating the Arab League deal, a resident said.

The International Committee of the Red Cross said on Wednesday it was pressing Syria for wider access to thousands of detainees arrested in pro-democracy protests.

Western sanctions and growing criticism from Turkey and Arab neighbours have raised pressure on Syria to end the bloodshed.

We are happy to have reached this agreement and we will be even happier when it is implemented immediately, said Prime Minister Sheikh Hamad bin Jassim al-Thani of Qatar, which leads an Arab League committee behind the plan agreed in Cairo.

China, which along with Russia, has resisted imposing U.N sanctions on Syria, welcomed the Arab League plan.

We believe this marks an important step towards easing the situation in Syria and the early launching of an inclusive political process with broad participation from all parties in Syria, Chinese Foreign Ministry spokesman Hong Lei said.

Despite the latest violence, Sami Baroudi, a political analyst at the Lebanese American University in Beirut, said it was too early to judge whether Syria would honour the agreement.

It will take at least a couple of days to see whether the intensity of violence is going down or up, or staying at the same level, Baroudi said. I wouldn't throw this initiative into the waste basket because nothing happened immediately.

After the deal was announced in Cairo, the United States reiterated its call for the Syrian president to quit.

The Arab League has not suspended Syria's membership or backed international intervention, as it did against Libya's Muammar Gaddafi's, who was toppled by NATO-backed rebels.

(Additional reporting by Erika Solomon in Beirut; Editing by Sophie Hares)