The Syrian Observatory for Human Rights said at least 29 people were killed in Homs alone, and warned the toll could rise as many of the dozens wounded had critical injuries.
State media reported the deaths of three soldiers and said a "terrorist group" blew up an oil pipeline in Homs.
Opposition groups and activists have said at least 200 civilians were killed in a "massacre" in Homs overnight Friday during a heavy tank and mortar bombardment by regime forces.
On Monday, the army also launched an assault on the Zabadani area near Damascus with heavy tank shelling, killing at least three people, the Britain-based Observatory said.
A resident of Homs told AFP the latest assault began shortly after 0400 GMT, with relentless barrages of rockets, mortar rounds and artillery shells.
"What is happening is horrible, it’s beyond belief," said activist Omar Shaker, reached by telephone as loud detonations were heard in the background.
"There is nowhere to take shelter, nowhere to hide," he said. "We are running short of medical supplies and we are only able to provide basic treatment to the injured."
One video posted on YouTube apparently showed a field hospital hit by shelling in the Baba Amro district and wounded patients lying on stretchers on the floor amid pools of blood and shattered glass.
Its authenticity could not immediately be verified.
Footage shot by a BBC undercover team in Homs showed buildings ablaze in rebel neighbourhoods as regime forces pounded them with heavy weapons. Plumes of white smoke billowed into the sky.
Damascus blamed the unrest in Homs on "terrorist gangs" using mortars.
The violence comes as Western powers seek new ways to punish Damascus amid growing outrage over Saturday’s veto by Russia and China of a UN Security Council resolution condemning Syria for a near 11-month crackdown on dissent.
The United States called the vote a "travesty."
On Monday, the US State Department said it had closed the American embassy in Syria and withdrawn remaining staff after Damascus refused to address security concerns.
Senior State Department officials told CNN that two embassy employees left by air last week and 15 others, including Ambassador Ford, departed overland via Jordan on Monday morning.
US President Barack Obama meanwhile said it was important to resolve the ongoing conflict diplomatically.
"It is important to resolve this without recourse to outside military intervention and I think that’s possible," he said in an NBC television interview.
Britain recalled its ambassador to Syria "for consultations," Foreign Secretary William Hague told parliament.
"We will use our remaining channels to the Syrian regime to make clear our abhorrence at the violence that is utterly unacceptable to the civilised world," Hague said.
French President Nicolas Sarkozy said after meeting German Chancellor Angela Merkel that he would call Russian President Dmitry Medvedev to discuss the international response to the crisis.
Neither France nor Germany, he said, would accept the "blocking" of action on Syria.
Britain was also seeking new ways of applying pressure on Syria through the UN General Assembly.
"Russia and China are protecting a regime that is killing thousands of people. We find their position incomprehensible and inexcusable," Prime Minister David Cameron’s spokesman said in London.
Russia and China both defended their vetoes on Monday, with Moscow condemning as "hysterical" the West’s angry reaction.
Foreign Minister Sergei Lavrov and Foreign Intelligence Service chief Mikhail Fradkov are due in Damascus on Tuesday, as news reports said the mission could try to push Assad to quit.
China called on both sides to the conflict to halt the violence that has claimed the lives of at least 6,000 people since March, according to rights groups.
Saturday’s double veto handed President Bashar al-Assad’s regime a "licence to kill," the opposition SNC said.
It urged Syrians around the world "to surround Syrian embassies and stage sit-ins outside them."
The SNC said the Syrian people tried in vain to find solutions to end the crackdown on protesters but the "genocide" in Homs shows the regime was "increasing the pace of its crime and repression."
Saudi Arabia called for "critical measures" on Syria and warned of an impending "humanitarian disaster" after the failure of the UN resolution.
The six-nation Gulf Cooperation Council, of which Riyadh is a member, is to meet on Saturday on Syria, on the eve of an Arab League ministerial meeting in Cairo.
