Syrian warships and security forces have killed 21 people in an assault on the port city of Latakia, activists said, even as world leaders demanded an immediate end to the ruthless crushing of dissent.

Security forces also surged into the Damascus suburbs of Saqba and Hamriya overnight, cutting off communications, firing shots and making arrests, said the Syrian Observatory for Human Rights on Sunday.

"Warships are attacking Latakia and explosions have been heard in several districts," the group said, adding that the main target was Ramleh suburb of the eastern Mediterranean port city.

Twenty-one people were killed and dozens of others seriously wounded, it said. The group said Palestinians also figured among the casualties of the assault on Ramleh, which is home to Palestinian refugees in Al-Ramal camp.

On Saturday, the military killed at least two more people and wounded 15 also in the Ramleh area of southern Latakia, a nerve centre of anti-regime protests, according to the advocacy group.