Syria's opposition warned Friday of a looming 'massacre' as it reported thousands of regime forces and militiamen encircled the protest hub of Homs for an expected final assault to crush dissent.

The Syrian National Council issued the alert ahead of nationwide protests following the main weekly Muslim prayers called for in support of a campaign of escalating strikes starting on Sunday.

In a statement, the SNC said President Bashar al-Assad's regime was using the pretext of what it called a "terrorist" attack on an oil pipeline to overrun Homs, which has already been besieged for months.

"The regime (is) paving the way to commit a massacre in order to extinguish the revolution in Homs," said the organisation, a principle umbrella group drawing together Assad's opponents.

Homs, an important 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," said the SNC.

"The regime also took a significant step... in burning oil pipelines in the neighbourhood of Baba Amr to blame what the regime calls 'armed gangs'; in an attempt to crush the peaceful uprising on the pretext of a war on terrorism."

Witnesses on the ground in the central city have reported a buildup of troops and pro-regime 'Shabiha' militiamen in armoured vehicles who have set up more than 60 checkpoints, said the opposition group.

"These are all signs of a security crackdown operation that may reach the level of a total invasion of the city."

"We warn of the consequences of committing such a crime that could result in a massive number of casualties," said the SNC.

"We hold accountable the regime, and behind it the Arab League and the international community of what could happen to innocent civilians in the next few hours or days, and the implications for the region as a whole in the near future.

"The Syrian National Council also calls on all relevant international organisations and human rights organisations to take immediate action to pressure the international forums to provide immediate protection to civilians in Homs in particular, and throughout Syria in general."

The Assad regime's crackdown on dissent since mid-March has hit Homs particularly hard and activists say a great number of defecting soldiers have set up camp there to protect the protest movement.

An explosion that tore apart a pipeline taking crude to an oil refinery in Homs from eastern Syria, in an attack the regime blamed on "armed terrorist gangs."

But the Local Coordination Committees (LCC), which organises anti-regime protests, accused Assad's government of deliberately destroying the pipeline which serves a region seen as staunchly opposed to his rule.

Activists reported at least 20 people killed in Thursday's violence alone, the majority of them in Homs. The regime's crackdown on dissent has killed more than 4,000 people in Syria, according to UN figures.

UN human rights chief Navi Pillay is due to address the Syria crisis on Friday and is expected to brief the UN Security Council by Tuesday at the request of France, Britain and German, diplomats said.

"It will be useful because it will allow the Security Council to examine its own responsibilities" in the crisis, said a UN diplomat speaking on condition of anonymity.

Diplomats said they notice signs of a shift in attitude by opponents of UN action against President Assad. However Western governments are waiting to see what impact Arab League sanctions have on Syria.

The Arab League is leaning on Iraq to persuade Syria to allow observers or else face more sanctions, but the regime has taken a defiant stance this week, with Assad himself denying responsibility for violence by his forces.

Syria's foreign ministry said in a statement that it is studying a response from the Arab League to conditions sought by Damascus to accept a delegation of monitors.

Pro-democracy activists, meanwhile, urged citizens to rally on Friday in support of a 'dignity strike which will lead to the sudden death of this tyrant regime.'

The LCC has called on citizens to strike from Sunday with sit-ins at work and the closure of shops and universities, before the shutdown of transportation networks and a general public sector strike.