Syrian forces rounded up dozens of people in Hama on Wednesday after shooting dead up to 22 people, activists said, and Amnesty International said Syria may have committed crimes against humanity in an earlier crackdown.

Tanks were still stationed outside Hama, which has seen some of the biggest protests against President Bashar al-Assad and was the site of a bloody crackdown against Islamist insurgents nearly 30 years ago.

But some of the tanks had been redeployed away from the city and a resident said security forces were now concentrated mainly around the headquarters of the ruling Baath Party, the police headquarters and a state security compound.

Ammar Qurabi, Cairo-based head of the Syrian National Human Rights Organisation, said the death toll from Tuesday, when gunmen loyal to Assad swept through the city, had risen to 22.

He said hundreds of people had been arrested.

State news agency SANA said one policeman was killed in a clash with armed groups who opened fire on security forces and threw petrol and nail bombs at them. It made no mention of civilian deaths but said some "armed men" were injured.

Syria has prevented most independent media from operating inside the country, making it difficult to verify accounts from activists and authorities.

Hama was emptied of security forces for nearly a month after at least 60 protesters were shot dead on June 3, but the security vacuum emboldened demonstrators and on Friday activists said at least 150,000 people rallied to demand Assad's downfall.

The next day Assad sacked the provincial governor and sent tanks and troops to surround the city, signalling a military assault similar to those carried out in other protest centres.

"Crimes against humanity"

In a report released on Wednesday, Amnesty International said the crackdown two months ago against one of those protest centres -- the town of Tel Kelakh near the border with Lebanon -- may have constituted a crime against humanity.

Urging the United Nations to refer Syria to the International Criminal Court, it said nine people died in custody after being captured during the operation in the town, close to the Lebanese border.

Describing a "devastating security operation", it said scores of men were rounded up, and most of them were tortured.

Some detainees told Amnesty they were beaten and held in the 'shabah' (ghost) position, tied by the wrists to a bar high enough off the ground to force them to stand on the tip of their toes for long periods.

A 22-year-old man told Amnesty he was tied up in the shabah position had electric shocks applied to his body and testicles during five days of detention in the provincial capital Homs.

"Amnesty International considers that crimes committed in Tel Kelakh amount to crimes against humanity as they appear to be part of a widespread, as well as systematic, attack against the civilian population," it said.

Syrian activists say security forces have killed more than 1,300 civilians since the unrest erupted 14 weeks ago. Authorities say 500 soldiers and police have been killed by armed gangs who they also blame for most of the civilian deaths.

Assad has responded to the protests with a mixture of repression and concessions, promising a political dialogue with the opposition. Preliminary talks on the dialogue are due to be held on Sunday.

But opposition figures refuse to sit down and talk while the killings and arrests continue, and diplomats say events in Hama will be a litmus test for whether Assad chooses to focus on a political or a military solution to the unrest.

Some residents sought to halt any military advance earlier this week by blocking roads between neighbourhoods with rubbish containers, burning tyres, wood and metal.

Activists, citing Hama residents, said the water supply had been cut and the city was operating an emergency reserve.

Assad's father, Hafez al-Assad, who ruled Syria for 30 years until his death in 2000, sent troops into Hama in 1982 to crush an Islamist-led uprising in the city where the armed wing of the Muslim Brotherhood made its last stand.

That attack killed many thousands, possibly up to 30,000, and one slogan shouted by Hama protesters in recent weeks was "Damn your soul, Hafez".

Reuters