Syrian forces have intensified their attack on rebel-held areas of the besieged city of Homs, activists say.
Hundreds of shells rained down on the city in a two-hour period, reportedly razing buildings and killing at least 30 people.
Activists also reported attacks by government forces in northern parts of the country, where another 33 died. The accounts cannot be verified.
The Red Cross has publicly called for a daily two-hour truce to allow in aid.
The future of Syria is set to be discussed at a meeting of world and Arab leaders in Tunisia on Friday.
But Russia, a key ally of the regime, says it will not attend.
Thousands have died in Syria in an 11-month uprising against the government.
The Baba Amr area of Homs, where several hundred fighters loyal to the opposition are believed to be hiding, has been under siege for two weeks.
Activists say government troops launched a barrage of shells on Tuesday morning, stepping up the attack.
Parts of the city are said to be running out of food and water.
Marie Colvin, a journalist for the UK's Sunday Times newspaper, said she had seen "sickening" scenes in Homs on Tuesday.
In an apartment which has been turned into a medical clinic she saw a "constant stream" of injured civilians, and she watched a baby die after shrapnel had pierced his body.
Opposition groups have reported that the army is reinforcing its presence around Homs, in preparation for a ground assault that rights groups have warned could turn into a massacre.
But the BBC's Jim Muir, monitoring events from neighbouring Lebanon, says it is unclear if the bombardment is the precursor to a much-feared ground assault.
The Syrian government said two weeks ago that it would wipe out "pockets of armed terrorists" in Homs.
International media organisations are heavily restricted in Syria, so it is impossible to verify the claims of either side.
An activist in hiding near Baba Amr 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," said the activist, named as Omar.
"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 brave Syrian people" needed support and the Friends of Syria meeting would show that the government of President Bashar al-Assad was increasingly isolated.
She criticised the positions of Russia and China, who have opposed Western and Arab peace initiatives, saying the two nations were "making the wrong choices".
Russia said it would not attend Friday's meeting because the Syrian government had not been invited, and accused the organisers of representing only one point of view.
Moscow has proposed that the United Nations send a special envoy to Syria to help co-ordinate the delivery of humanitarian aid.
The Red Cross (ICRC) said it had been negotiating with both sides to call for a regular "humanitarian ceasefire".
"It should last at least two hours every day, so that ICRC staff and Syrian Arab Red Crescent volunteers have enough time to deliver aid and evacuate the wounded and the sick," said ICRC President Jakob Kellenberger in a statement.
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.