Heavy artillery fire has been rocking Homs, as Syrian troops step up an assault on the restive city.
A BBC correspondent there describes almost constant blasts, in the fiercest attack in the 11-month uprising.
US President Barack Obama said it was important to resolve the conflict without outside military intervention.
Meanwhile, Russia and China defended their veto of a UN draft resolution criticising Syria - a move that angered opponents of President Bashar al-Assad.
Later the US State Department said it had closed its embassy in Damascus and pulled out all remaining staff because of security concerns.
Washington had warned in January that it would close the embassy if the government did not step up security.
'Not safe at all'
Homs, one of the main centres of resistance to Mr Assad's rule, has been under attack from government forces for several days.
Shelling resumed shortly after daybreak on Monday, says BBC's Paul Wood who has managed to get into the city, and hundreds of shells and mortars have been fired throughout the day.
Eyewitness Danny Abdul Dayem told the BBC the army was using rockets for the first time, with more than 300 falling on his locality since dawn.
"It's not safe at all, a rocket could land in this house right now," he said
Some rebels fighters have been firing automatic weapons in return, in what our correspondent calls a futile gesture.
The rebels claim that the shelling has hit a field hospital in the Baba Amr district, causing casualties. However, our correspondent says this is impossible to verify.
The facility is treating dozens of people wounded in previous assaults on Homs.
Mr Dayem said only one field hospital with four doctors was still operating in the city, and it was virtually impossible to get additional medication without being shot.
Another anti-government campaigner told the BBC the government was also using helicopters and tanks in the assault.
Activists say at least 40 people were killed on Monday.
Syrian state TV said "terrorist gangs" had blown up buildings in Homs.
The state-run Sana news agency reported that an oil pipeline near the city had been hit by an explosion on Monday. It also blamed "terrorists".
Both Syrian media and activists are also reporting clashes in the northern city of Idlib and the town of Zabadani, north-west of Damascus.
The government says it is fighting foreign-backed armed groups. Thousands of former army soldiers have defected to the rebel side, forming the Free Syrian Army.
The BBC's Jim Muir in neighbouring Lebanon says the conflict is beginning to look increasingly like a civil war with dangerous sectarian overtones.
The uprising is largely rooted in poorer sections of the Sunni community, our correspondent says, while the government draws its support mostly from Alawites, Christians and other minorities fearful of an Islamist takeover.
The Syrian opposition says Saturday's veto by China and Russia of a UN draft resolution condemning the crackdown will encourage the government to act without restraint.
President Obama vowed to apply sanctions and put pressure on Mr Assad.
"I think it is very important for us to try to resolve this without recourse to outside military intervention. And I think that's possible," he said in an interview for the NBC network's Today programme broadcast on Monday.
He added that a negotiated solution was possible and said the US was "relentless" in demanding that Mr Assad leave power.
"The Assad regime is feeling the noose tightening around them," he said. "We're going to just continue to put more and more pressure until hopefully we see a transition."
UK Foreign Secretary William Hague described the Russian and Chinese vetoes as "a grave error of judgement".
French President Nicolas Sarkozy said he would discuss the situation in Syria with his Russian counterpart Dmitry Medvedev later on Monday.
"France and Germany will not abandon the Syrian people," Mr Sarkozy said after a meeting in Paris with German Chancellor Angela Merkel. "We will not accept that the international community remains blocked."
However, Russian Foreign Minister Sergei Lavrov said the resolution, drafted by Arab and European countries, would have meant taking sides in a civil war.
Speaking in Bahrain on Monday, he said encouraging "armed extremist groups" would only cause more casualties, and said Moscow supported peaceful dialogue in Syria.
Mr Lavrov is due to travel to Damascus on Tuesday for talks with President Assad.
The Chinese government also defended its veto. It said the draft resolution would only have complicated matters, and said Beijing sought to "avoid the scourge of armed conflict".
Human rights groups and activists say more than 7,000 people have been killed by Syrian security forces since the uprising began last March.
The UN stopped estimating the death toll in Syria after it passed 5,400 in January, saying it was too difficult to confirm.
President Assad's government says at least 2,000 members of the security forces have been killed fighting "armed gangs and terrorists".