Damascus: Syrian military vessels joined an assault that killed more than 20 people on Monday in the city of Latakia, activists said, the first attack from the sea since an anti-regime revolt erupted in March.

The latest bloodshed came a day after world leaders demanded an immediate end to the ruthless crushing of dissent in Syria.

The Syrian Observatory for Human Rights said in a statement that at least 20 people were killed and dozens of others wounded in the Mediterranean port city, several of them critically.

The vessels opened up with heavy machine-guns, it said.

In May, during an army operation in Banias, another port, naval ships patrolled offshore but without opening fire.

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.

Ships are "attacking Latakia and explosions have been heard in several districts," the group said earlier, adding that the main target was the suburb of Ramel in the port city.

The group said Palestinians also figured among the casualties of the assault on Ramel, which is home to a Palestinian refugee camp.

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