The Syrian Observatory for Human Rights said 21 people were killed and dozens of others wounded on Sunday, several of them critically.
"Warships are attacking Latakia and explosions have been heard in several districts," the Britain-based group said earlier, adding that the main target was the Ramleh district.
On Saturday, the military killed at least two people and wounded 15 others in the Ramleh area of southern Latakia, a nerve centre of anti-regime protests, according to the group.
"Large numbers of residents, especially women and children" have fled Ramleh, the scene of mass protests calling for the fall of President Bashar al-Assad's regime, it said.
The Syrian Observatory said landline telephones and Internet connections with the eastern Mediterranean port city were cut.