The US has closed down its embassy in Syria and pulled all of its employees out of the country, amid reports that the Syrian military is intensifying its assault on the central city of Homs.
The UK also announced on Monday it would recall its ambassador to Syria for what it called consultations.
Activists and witnesses said the Syrian army had stepped up its attacks on opposition fighters in Homs after a UN Security Council resolution, aimed at ending the bloodshed in the country, was blocked by Russia and China on Saturday.
Abu Abdo Alhomsy, an activist of a revolutionary council in Homs, described to Al Jazeera an attack on Monday on the city as live pictures showed smoke billowing into the sky.
"It is horrible right here," Alhomsy said. "Rockets are falling. There are massive explosions that shook buildings. We don't know really what to do.
"Its a massive attack - a new massacre is happening here. Nobody can go out, we don't know how many homes have been hit or how many people died."
The UK-based opposition group, the Syrian Observatory for Human Rights (SOHR), said at least 29 civilians were killed and dozens more wounded.
But it said the death toll was likely to rise because many of the wounded had suffered critical injuries.
The shelling, which began at dawn, was mainly targeting the neighbourhoods of Khaldiyeh, Bab Amr, Inshaat and Bab Sbaa, the SOHR said.
Video received by Al Jazeera from opposition activists on Sunday showed the apparent devastation caused by a military offensive in the Bab Amr neighbourhood of Homs.
A video posted by activists on YouTube apparently showed a field hospital hit by the shelling in Bab Amr. Wounded patients could be seen lying on stretchers on the floor amid pools of blood and shattered glass. Bodies of dead people with horrific injuries were also shown inside the hospital.
The authenticity of the video could not immediately be verified.
Syrian state television has accused "armed gangs" of being behind the latest violence in Homs.
Activists and witnesses said the army had been shelling Bab Amr "indiscriminately" since Sunday morning.
The video images appear to show people who have been shot and hit by shrapnel, including heavily injured young children.
Speaking to Al Jazeera, Danny Abdul Dayem, a resident of Homs, said: "It has been terrible. There is non-stop bombing with rockets, mortar bombs and tank shells. There were more than 50 people injured in Bab Amr today.
"I saw with my own eyes kids with no legs, and a kid who lost his whole bottom jaw. It is terrible."
In the northeast of the country, army deserters destroyed a military control post early on Monday, killing three officers and capturing 19 soldiers in the process, the SOHR said.
The fighting happened in the village of Al Bara in the Idlib region, it said.
It said none of the army deserters involved was killed and that the regular army post was completely destroyed.
The death toll in Syria rose to at least 88 people over the weekend - one of the bloodiest since the uprising against Bashar al-Assad's government erupted almost 11 months ago.
Opposition groups say at least 6,000 people have now been killed in Syria.
The US and British diplomatic protests come two days after Russia and China vetoed the UN resolution that backed an Arab plan calling on Assad to quit.
Robert Ford, the US ambassador, and 17 other US officials left Syria and were expected to travel back to the US. Ford informed Syrian authorities of the decision to leave earlier on Monday, state department officials said. Two diplomats left by air and the others went overland to Jordan.
The state department said two weeks ago that it would close the embassy unless Assad's government better protected the mission, citing safety concerns about embassy personnel and a recent series of car bombs.
In-depth coverage of escalating violence across Syria
"We have been relentless in sending a message that it is time for Assad to go," President Barack Obama said during an interview with NBC. "This is not going to be a matter of if, it's going to be a matter of when.''
Meanwhile, in London, William Hague, the UK foreign secretary, told parliament: "I have today recalled to London our ambassador in Damascus for consultations."
He said Britain and other nations would consider a resolution at the UN General Assembly, in the absence of a Security Council resolution on Syria. Britain would increase pressure on Syria through the European Union, he said.
"We have already agreed 11 rounds of EU snactions and will hope to agree further measures by the [EU] Foreign Affairs Council on February 27," Hague said.
"This is a doomed regime as well as a murdering regime," he said. "There is no way it can recover its credibility internationally."
For their part, Angela Merkel, the German chancellor, and Nicolas Sarkozy, the French president, spoke with one voice on Syria after holding talks in Paris on Monday.
"Germany and France will not let the Syrian people down on the Syrian issue," Sarkozy said, announcing plans for a Libya-style international contact group on Syria.