Syrian security forces have stepped up their deadly siege in the port city of Latakia, with gunboats joining the assault.

Activists say government forces killed at least 19 people in the city on Sunday, a day after an assault with armored vehicles left at least two people dead. They say gunboats with machine guns fired on coastal districts Sunday.

The government operation comes after protesters massed in the port city renewing calls for President Bashar al-Assad's resignation.

Friday's demonstration was one of many that took place across Syria. Activists and witnesses say security forces killed at least 19 people that day as the government widened its crackdown on dissent.

Mr. Assad is facing growing international condemnation for the crackdown.

On Saturday, U.S. President Barack Obama and Saudi Arabia's King Abdullah called for an immediate end to the government's violence against demonstrators.

In a separate phone call, Mr. Obama and British Prime Minister David Cameron also agreed on the need for an immediate end to the bloodshed.

Also, Canada announced it is expanding economic sanctions against Syria, and one of the world's largest Muslim inter-governmental organizations urged Syria to exercise restraint. The Organization of Islamic Cooperation also offered to play a role in dialogue in Syria.

Meanwhile, the United Nations Security Council plans to meet Thursday to consider further action against the Syrian government.

Details of events in Syria are difficult to independently verify because the government allows very few foreign news reporters into the country and restricts their movements.

Rights groups and activists say at least 1,700 people have been killed since the start of the government's crackdown.