Armoured vehicles entered the port city of Latakia and a village near Lebanon Saturday, activists said, causing residents to flee as the West seeks ways to pressure Damascus to end the violence.
"Military vehicles including tanks and armoured personnel carriers converged on the southern district of Al-Ramleh" in Latakia, a statement by the Britain-based Syrian Observatory for Human Rights said.
"At 10:30 am heavy gunfire could be heard" in Al-Ramleh, which was at the heart of a "large demonstration calling for the fall of (President Bashar al-) Assad's regime."
The Observatory said the arrival of troops sparked the exodus of a large number of residents, especially women and children.
It also reported a "wave of arrests" in Latakia on Thursday.
An activist in the central region of Homs said troops backed by two tanks entered the village of Jussiyeh which borders Lebanon, triggering a stampede across the frontier and to neighbouring areas.
Military vehicles, meanwhile, swooped on the town of Qusayr, also in Homs province, where security and intelligence services launched an arrest campaign.
"No one was spared. Not even women or children," said the Observatory.
Security forces backed by tanks have been crushing dissent city by city and town by town since pro-democracy protests erupted in mid-March.
The Observatory says 2,150 people have been confirmed dead since then -- 1,744 civilians and 406 members of the security forces.
Activists said at least 16 people were killed on Friday when security forces opened fire on thousands of anti-regime protesters rallied in flashpoint cities after the Ramadan weekly prayers.
State television said "two security agents were shot dead by armed men in Douma," a suburb of the capital.
The UN Security Council is to hold a special meeting next Thursday to discuss human rights and the humanitarian emergency in Syria, diplomats at the United Nations said.
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In a Twitter statement, France's UN mission said UN Human Rights Commissioner Navi Pillay and UN under secretary for humanitarian affairs Valerie Amos, will brief the meeting.
As the West grapples with ways to pressure Damascus into ending the bloodshed, US Secretary of State Hillary Clinton urged countries to stop trading with Syria.
"We urge those countries still buying Syrian oil and gas, those countries still sending Assad weapons... to get on the right side of history," Clinton told reporters.
In an interview with CBS News, she suggested that China and India impose energy sanctions on Syria, and urged Russia to stop selling arms to Damascus.
She also urged the Europeans to impose energy sanctions.
"President Assad has lost the legitimacy to lead and it is clear that Syria would be better off without him," Clinton told a news conference with Norwegian Foreign Minister Jonas Gahr Stoere.
But she stopped short of explicitly urging Assad to step down -- a call which US officials have said President Barack Obama's administration has decided to make, although it has not finalised the timing.
A dual nationality Canadian engineer on Friday accused Ottawa of "indirectly financing" the Damascus regime.
Abdullah Almalki, 40, was arrested in Syria in 2002 on the basis of information provided by Canadian authorities who suspected him of terrorism. He returned to Canada in 2004 and was cleared of all accusations.
"The oil and gas revenues do not go to the benefit of the Syrian people -- it goes to the Assad regime," he told CBC at a demonstration in Ottawa.
"Nowadays they're being used to supply the killing machine, to supply the rounds, the bullets and the salaries of the thugs of the government who are killing day in, day out."
The Canadian oil firm Suncor has invested some $1.2 billion in a partnership with Syria's state-run General Petroleum Corporation to exploit oil and gas fields in central Syria.
Clinton also said the US ambassador to Damascus, Robert Ford, delivered a "clear message" when he met Syrian Foreign Minister Walid Muallem on Thursday.
"Immediately stop the violence, withdraw your security forces, respond to the legitimate aspirations of the Syrian people for a democratic transition in concrete and meaningful ways," she said, reading out the message.