Rebel-held areas of the central Syrian city of Homs have come under intense bombardment from the Syrian army.
Anti-government activists report that at least 12 people have been killed so far on Tuesday and 100 injured. This cannot be independently confirmed.
Activists have been warning of additional government forces around Homs, ahead of a possible ground assault on the city.
Thousands have died in Syria in an 11-month uprising against the government.
The area of Baba Amr, which has been under siege for two weeks, endured several hundred shells over a period of one or two hours, says the BBC’s Jim Muir, who is monitoring events from neighbouring Lebanon.
He says it is not clear whether the shelling is continuing as communications with residents of Homs are very poor.
Reports from the city say the field hospital has been hit, and video footage appears to show tanks moving through a neighbouring district of Homs.
Our correspondent says it is unclear if the bombardment is the precursor to a much-feared ground assault on Baba Amr.
Human rights groups say that could lead to a massacre and have called for intervention.
The Syrian government said two weeks ago that it would wipe out “pockets of armed terrorists” – its term for the rebel fighters – implying that a ground assault on Baba Amr is on its agenda.
The International Committee of the Red Cross (ICRC) said on Monday that it was trying to negotiate a short humanitarian ceasefire so that food and medical supplies could be brought into the areas of Syria worst affected by the violence.
It remains unclear when or if that might happen.
‘Defenceless civilians’
An activist in hiding near Baba Amr, Omar, told the BBC that it was no longer safe to stay there.
“I’m trying to leave the area because of the gunfire and heavy shelling, which has rocked the city,” he said.
“This is a large-scale military assault on defenceless civilians.”
The casualties included two children, one of whom was six years old, he said.
Western and Arab countries are preparing to attend a meeting on Syria in the Tunisian capital Tunis this Friday.
The US Secretary of State Hillary Clinton said the Friends of Syria meeting would show that the government of President Bashar al-Assad was increasingly isolated and that “the brave Syrian people” needed support.
She criticised the positions of Russia and China, saying they were “making the wrong choices”.
Russia and China vetoed a UN Security Council resolution condemning the violence and backing an Arab League plan aimed at ending the conflict in Syria.
They will not be attending Friday’s meeting.
Russia proposed on Monday that the United Nations send a special envoy to Syria to help co-ordinate the delivery of humanitarian aid.
‘Help arm fighters’
US Senator John McCain has said that Washington and its allies should find a way to help arm the opposition fighters.
“It is time we gave them the wherewithal to fight back and stop the slaughter,” he said on a visit to the Egyptian capital, Cairo.
Human rights groups believe more than 7,000 people have been killed since the uprising began.
The Syrian government says at least 2,000 members of the security forces have died fighting militants.
It is pressing on with its plans for a referendum on a proposed new constitution on Sunday, which it regards as the centrepiece of its reform programme.
-BBC
