Syrian forces killed 68 civilians Tuesday as they blitzed the city of Homs and a village in Idlib province, monitors said, as the United States expressed support for a humanitarian ceasefire proposed by the Red Cross.

In the most significant incident, at least 33 people were killed in the village of Abdita in the northwestern province of Idlib, the Syrian Observatory for Human Rights said.

Another 31 were killed in Homs province, including 21 people who died in "intensive shelling" that targeted Homs city's Baba Amr neighbourhood, with the Khaldiyeh and Karm al-Zaytoun districts also blasted.

And despite a plea by activists to allow women and children to flee Homs' besieged Baba Amr neighbourhood, more troops were sent to the outskirts of the restive city, with activists expressing fear they were preparing to storm it.

Homs-based activist Hadi Abdullah of the General Commission of the Syrian Revolution told AFP "large reinforcements were heading to Homs."

"We counted at least 150 shells crashing in Baba Amr within two hours this morning. We gave up counting afterwards," he said.

Omar Shaker, another activist, told AFP the neighbourhood had "no electricity, nor fuel," and that "snipers have hit water tanks," rendering the situation "bad beyond imagination."

Human Rights Watch emergency director Peter Bouckaert told AFP the watchdog had confirmed the use of Russian-made 240 mm mortars in Homs, which has been under assault for 18 days.

"We have little doubt that those extremely powerful mortars are being fired by the regime forces into civilian neighborhoods of Homs. We are talking about a 250-pound mortar round that can only be fired from a heavy specialised armoured vehicle and it requires a nine person crew to operate," he said.

AFP was not able to verify the death toll nor the reports of shelling, as foreign reporters are given only limited access within the country.

Security forces also opened fire to disperse a sit-in by 2,500 students at the University of Aleppo, the northern city that until recently been spared anti-regime demonstrations.

And secular groups demonstrated outside parliament against an article in the draft constitution that would require the president to be a Muslim.

The International Committee of the Red Cross called for a daily truce of two hours in Syria so it can deliver vital aid to afflicted areas, after saying a day earlier it was in talks with both sides to halt the violence.

The head of the rebel Free Syrian Army, Colonel Riyadh al-Asaad, welcomed the call but voiced doubts that the "criminal" regime would commit.

And the UN under secretary general for humanitarian affairs, Valerie Amos, called on Syria to allow aid groups unimpeded access to the country.

"This is a major human rights crisis that is now moving into significant humanitarian consequences," Amos said.

In Washington, White House press secretary Jay Carney said "we support calls for cease-fires to allow for the provision of humanitarian supplies to Syrians who desperately need it."

"Reprehensible actions taken by the Assad regime have led us to a situation where basic supplies, humanitarian supplies are very scarce."

And although top US military officer, General Martin Dempsey, has said it was "premature" to arm Syria's opposition, top Republican Senator John McCain called again for the outgunned rebels to be supplied with weapons.

But State Department spokeswoman Victoria Nuland said "from our perspective, we don't believe that it makes sense to contribute now to the further militarisation of Syria.

"What we don't want to see is the spiral of violence increase. That said, if we can't get Assad to yield to the pressure that we are all bringing to bear, we may have to consider additional measures," she said.

US Secretary of State Hillary Clinton said Syria was increasingly under pressure.

A Friends of Syria meeting in Tunis on Friday will "demonstrate that Assad's regime is increasingly isolated and that the brave Syrian people need our support and solidarity," she said.

The meeting "will send a clear message to Russia, China and others who are still unsure about how to handle the increasing violence but are up until now, unfortunately, making the wrong choices," she added.

Russia announced that it will not take part in the meeting because it was being convened "for the purpose of supporting one side against another in an internal conflict," the foreign ministry said.

China, meanwhile, refused to commit to attend the meeting.

The Friends of Syria group will meet for the first time after being created in response to a joint veto by China and Russia of a UN Security Council resolution condemning the bloody crackdown.

It is backed by members of the European Union as well as some Arab nations and the United States.

"China has received the relevant invitation," foreign ministry spokesman Hong Lei said. "The Chinese side is currently researching the function, mechanism and other aspects of the meeting."

In other developments, several hundred people demonstrated in the Gaza Strip calling for the downfall of Assad, whom they called a butcher and a criminal.

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