"Israeli warplanes launched strikes" targeting a series of sites including air defence units and "surface-to-surface missile depots", the Syrian Observatory for Human Rights said.
It said 18 raids "targeted strategic locations on the Syrian coast", added the Britain-based Observatory, which relies on a network of sources inside the country.
It called the raids "the heaviest strikes in Syria's coastal region since the start of strikes in 2012".
Tartus province also has a naval base belonging to Russia, a close ally of president Bashar al-Assad whom Islamist-rebels ousted just over a week ago after capturing swathes of the country in a lightning offensive.
In the village of Bmalkah in the hills above Tartus, an AFP journalist saw roads filled with shattered glass and shreds of roller doors.
The force of blasts had stripped the leaves of olive trees in groves surrounding the village, and smoke still rose from nearby hillsides.
Residents told AFP that explosions began shortly after midnight and continued until almost 6:00 am (0300 GMT).
"It was like an earthquake. All the windows in my house were blown out," said 28-year-old Ibrahim Ahmed, an employee in a legal office.
Clean-up crews sawed at fallen trees that had blocked the road to the next community. They also swept up missile and shell parts, even as the valley echoed to more blasts as pockets of stockpiled munitions caught fire.
"The village did not sleep last night. The kids were crying," said one middle-aged man in a blue sweatshirt who refused to give his name.
"Most of the people had already left their homes towards the city, now they have lost their houses," he added.
At a nearby military complex, smoke billowed from arched concrete bunkers cut into the hillside, and secondary explosions threw out shrapnel that fell among the trees.
Broken parts of mortars, rockets and missile launch tubes littered the hillsides.
According to the Observatory, 473 Israeli strikes have targeted military sites in Syria since the rebel alliance toppled Assad on December 8.
