"It was an intended terrorist attack that failed," said police spokesman Micky Rosenfeld.

One barrel was found on the sands of the city of Ashkelon and the other a few miles to the north at Ashdod. And yesterday, the third "barrel bomb" was found at Palmachim beach, 15 miles south of Tel Aviv.

A senior Palestinian source claimed that the explosives - from the Gaza Strip - were to be used to attack an oil rig in southern Israel.

Police are also checking the possibility that the barrels were intended to explode next to a ship or among beachgoers.

A bomb squad defused one barrel while the explosives in the second blew up while a robot was inspecting them.

After police had spent Monday night combing the closed the beaches for more devices, a rocket from Gaza exploded in an open field in southern Israel, causing no injuries or damage.

Maritime authorities warned sailors and fishermen to be alert to possible dangers in the waters.

Surfers were ordered out of the water and the public was told to keep away.

In Ashdod, a small crane-like robot crawled along the beach, prodding suspicious objects.

"The beach is closed, boats are patrolling the waters and helicopters are in the air looking for more explosive barrels," said restaurant owner Lee Voldberg, of Ashdod.

Meir Chakmon, of Ashkelon police, said: "We think there is a possibility that new explosive devices will surface.

"We don't know how many are in the sea."

Three terror group in Gaza claimed responsibility for the attack, saying that they sent a total of eight explosives to Ashkelon and detonated them by remote control.