Syrian forces have killed another 24 people in flashpoint cities, rights activists said, as the opposition warned of an impending "massacre" by regime troops ringing the central protest hub Homs.

Four children were among 24 people killed when regime forces opened fire in several cities across the country after the weekly Muslim prayers, the Syrian Observatory for Human Rights reported.

The Observatory said 11 civilians were killed in and around Homs, while five died near Damascus, two in Deraa, cradle of anti-regime protests since March, four in the restive city of Hama and two in the north-western province of Idlib.

Pro-democracy activists had called on citizens to turn out in support of a "dignity strike... which will lead to the sudden death of this tyrant regime."

The opposition Syrian National Council (SNC) warned of a looming bloody final assault on Homs using the pretext of what the regime had called a "terrorist" attack Thursday on an oil pipeline.

"The regime [is] paving the way to commit a massacre in order to extinguish the revolution in Homs," said the coalition SNC.

Witnesses in Homs, already besieged for months, have reported a build-up of troops and pro-regime "shabiha" militiamen in armoured vehicles who have set up more than 60 checkpoints, the SNC said.

"These are all signs of a security crackdown operation that may reach the level of a total invasion of the city", it added, calling for international organisations to take action.

The United States and Britain separately voiced concern over the bloodletting in Homs, where 11 of Friday's deaths occurred, and Washington urged president Bashar al-Assad's regime to allow independent monitors into the country in line with an Arab League peace plan.

US State Department spokeswoman Victoria Nuland said that Mr Assad would be responsible for any further deaths and expressed Washington's deep concern for developments in Homs.

"There are reports today that the government may be preparing a very serious new assault on the city of Homs in a very large-scale way," Ms Nuland said.

Damascus, which blames "armed terrorist gangs" for the violence, meanwhile appealed to the international community to help it find an "honourable exit" to the crisis, notably by stopping the flow of weapons into Syria.

Homs, an important central junction city of 1.6 million residents mainly divided along confessional lines, is a tinderbox of sectarian tensions that the SNC said the regime was trying to exploit.

"The regime has tried hard to ignite the sectarian conflict using many dirty methods, which have included bombing and burning mosques, torturing and killing young men, and kidnapping women and children," the SNC said.

Mr Assad, speaking to the American ABC News this week, denied responsibility for the bloodshed. He drew a distinction between himself and the military, saying only a "crazy" leader would kill his own people.

But Syrian foreign ministry spokesman Jihad Makdisi told a news conference in Damascus Friday evening that ABC had "distorted" Mr Assad's comments.

"It deliberately deformed the president's words... by airing videos (of violence) to incite" action against Syria, said Mr Makdisi.

He said Mr Assad was "appalled by the ongoing violence" and has promised "accountability," adding that Damascus needed international help to stop the flow of weapons into the country as a way to end the unrest.

UN chief Ban Ki-moon and UN human rights chief Navi Pillay both hit back at Mr Assad's claim that the United Nations had inflated its figures for the number of people killed in the crackdown.

"All the credible information is that more than 4,000 people have been killed by the government forces," Mr Ban said.

AFP