|A large anti-government protest was held in Latakia on Friday [EPA]
Five civilians have reportedly been killed in Syria, as troops backed by tanks and armoured vehicles entered several cities.
Activists said the heaviest assault on Saturday was on the coastal city of Latakia.
The UK-based Syrian Observatory for Human Rights (SOHR) said tanks rolled into the al-Ramel neighbourhood amid intense gunfire that sent many residents fleeing the area.
Later in the day, shooting and explosions were heard in another neighbourhood, Slaibeh, according to the SOHR and the Local Co-ordination Committees (LCC).
Also on Saturday, scores of security agents and pro-government thugs, known as shabiha, entered the town of Qusair near the border with Lebanon and several nearby villages, arresting scores of residents, Rami Abdel Rahman, chief of SOHR, said.
'Violence must end'
Canada said on Saturday it had expanded sanctions on Syria to protest the government's brutal crackdown on protests.
The new sanctions include travel bans on four officials and freezing the assets of the state-run Commercial Bank of Syria, and Syriatel, the country's largest mobile phone company.
The US imposed sanctions on the two firms earlier in the week, and has joined European allies in sanctioning top officials close to Assad.
Canada's sanctions came after US President Barack Obama spoke with the leaders of Saudi Arabia and the UK and all three called for an immediate end to the Syrian government's crackdown on protests.
Obama and Saudi King Abdullah "expressed their shared, deep concerns about the Syrian government's use of violence against its citizens," the White House said in a statement.
"They agreed that the Syrian regime's brutal campaign of violence against the Syrian people must end immediately, and to continue close consultations about the situation in the days ahead."
Similar language was used in a statement after a separate Obama conversation with British Prime Minister David Cameron.
In-depth coverage of escalating violence across Syria
Tens of thousands of people rallied in cities across the country on Friday in protest against the government and at least 17 people were reported killed.
The protests have grown dramatically over the past five months, driven in part by anger over the government's bloody crackdown in which rights groups say at least 2,000 civilians have been killed across the country.
The government has justified its crackdown by saying it is dealing with terrorist gangs and criminals who are fomenting unrest.
A Latakia resident speaking to Al Jazeera rejected the government's claims.
"There are no armed gangs here," he said. "We have been demonstrating peacefully for the last three months."
He said the army and security personnel together with regime thugs were shelling residential neighbourhoods in the city on Saturday.
Syrian authorities have expelled most independent journalists since the five-month-old uprising against Assad began, making it difficult to verify reports from both sides.