At least two gunboats were said to be taking part in the Latakia operation alongside 20 tanks and personnel carriers, the BBC reports. Areas of the Sunni-majority city that have seen major protests against the rule of President Bashar al-Assad were reportedly being targeted.
Reuters reports that 24 people were killed in the violence Sunday.
"I can see the silhouettes of two grey [naval] vessels. They are firing their guns and the impact is landing on al-Ramleh, al-Filistini and al-Shaab neighborhoods," a witness told Reuters.
Residents told the Guardian that machine guns were fired from at least one ship, and from several armored vehicles.
The Syrian Observatory for Human Rights said 32 civilians were killed and dozens more wounded Sunday.
International journalists face major restrictions in operating in Syria, and it is hard to independently verify reports.
Amateur video footage posted on YouTube showed what appeared to be armored personnel in the streets of Latakia.