The United States and United Kingdom increased their diplomatic pressure on Syria Monday, the latest signs that Western nations have lost patience with President Bashar Assad's regime.

The U.S. announced the closure of its Syrian embassy and pulled its diplomats from the country on a day that saw another 40 people killed in the opposition stronghold city of Homs, where government forces have ramped up their crackdown on protesters in recent days.

Later Monday, Britain announced it has recalled its ambassador from Syria for consultations, as did the Italian Foreign Ministry, which expressed its "strongest condemnation and the indignation of the Italian government over the unacceptable acts of violence perpetrated by the regime of Damascus against the civilian population."

The latest offensive began Saturday, hours before a UN Security Council resolution calling for Assad's ouster was vetoed by Syria's allies, China and Russia.

The measure, which had the support of both UN member states and the Arab League, was aimed at stemming the tide of violence that has so far killed more than 5,400 people since the beginning of the uprising and the regime's bloody crackdown.

"We have been relentless in sending a message that it is time for Assad to go," U.S. President Barack Obama told NBC. "This is not going to be a matter of if, it's going to be a matter of when."

While Obama said U.S. officials will continue to pressure Assad to step down, he said regime change should not be brought about by military intervention.

British Foreign Secretary William Hague said Monday officials are using multiple channels to express Britain's "abhorrence" at the violence.

"This is a doomed regime as well as a murdering regime," Hague said. "There is no way it can recover its credibility internationally."

Hague said the British government will continue to support efforts by the Arab League to foster democracy in Syria, and also increase its communication with members of the opposition.

Also Monday, Canada's official Opposition called on the Harper government to follow Britain's move and recall the Canadian ambassador.

NDP foreign affairs critic Helene Laverdiere said the move would send a message to the Assad regime.

"It's not a question of cutting diplomatic ties completely," Laverdiere said Monday. "Our position does not go as far as that of the United States."

NDP MP Paul Dewar, a party leadership candidate, said the time for influencing Syria through backchannel means is over.

"At some point you have to take action, particularly with a regime as repugnant as this one," he said on CTV's National Affairs Monday.

However, Chris Day, a spokesperson for Foreign Affairs Minister John Baird, said the government has no plans to shutter Canada's mission in Damascus, despite having called home a number of diplomats last week.

Day told The Canadian Press that ambassador Glenn Davidson will remain at his post with just a small number of support staff.

"The safety of Canadian staff in Syria is our number one priority," Day said. "Canada has extremely strong views about the abhorrent actions of the current regime. And our Ambassador continues to deliver those messages at the highest levels."

After the UN vote, U.S. Secretary of State Hillary Rodham Clinton called for "friends of democratic Syria" to unite in efforts to oust Assad, which could lead to a contact group of friendly nations that will offer assistance to the opposition.

Clinton called Saturday's UN vote "a travesty."

Violence intensifies

Meanwhile, the latest deaths in Homs were reportedly caused by Syrian army shells that slammed into a medical clinic and a residential neighbourhood. More than a dozen others were killed in other parts of the country, activists said.

CTV Middle East Bureau Chief Martin Seemungal said it is difficult to confirm not only what is happening in Syria, but also why, given that there are few independent observers or journalists inside the country.

"So we hear what is going on from the activists, we hear what the government has to say, but it's hard to get a good sense of what is going on, an independent view," he told CTV News Channel in an interview from Jerusalem.

The regime has denied it is attacking Homs, alleging that armed members of the opposition, the so-called Free Syrian Army, are actually firing shells to make it appear as though the government is attacking the city, Seemungal reported.

The state-run news agency also said opposition gunmen had killed three soldiers in Homs and captured soldiers at a checkpoint in the Jabal al-Zawiyah region, which borders Turkey.

Activists on the ground say government forces ramped up their bombardment of Homs after these attacks on government troops.

"As of 6:30 this morning, the shelling intensified with a rate of one shell every two minutes," Baba Amr activist Omar Sheker told The Associated Press on Monday.

Seemungal said the Syrian army had pressed into Homs in an effort to reassert control there. The move came amid reports the Free Syrian Army had taken charge of the city considered the epicenter of the country's anti-Assad movement.

On Saturday, as many as 200 people were killed in Homs, activists said, making it the highest death toll for a single day since the uprising began in March.

In Cairo, the Arab League spoke out against the latest violence. Arab League head Nabil Elaraby said he was "extremely alarmed and concerned" at reports the government was using heavy weapons against citizens.

It was the League's proposal for a transfer of power from the Assad regime that formed the basis of the failed UN resolution.

The U.S. ambassador to the UN, Susan Rice, told CBS that China and Russia "will come to regret" their votes.

On Monday, China said it was compelled to veto the resolution because the vote was called before all parties could work out their differences. For China, the resolution presupposed the outcome of any dialogue between the government and the opposition in Syria and put undue pressure on Assad's regime.

"On the issue of Syria, China is not sheltering anyone nor do we intentionally oppose anyone. We uphold justice and take a responsible attitude," Chinese Foreign Ministry spokesperson Liu Weimin said Monday.

The Russian foreign minister is scheduled to meet with Assad on Tuesday, but it is unclear what, if any, impact that will have on the ongoing violence or the efforts to oust Assad.