A wave of floating bombs designed to wash up on Israel's beaches were released in a revenge attack by Palestinian militants for the death of a top Hamas member.
Two bombs have been discovered but the militants claimed more were still in the Mediterranean and Israeli police cordoned off a swathe of the country's coastline while they searched for them.
Prime Minister Benjamin Netanyahu said Israel would "respond" to the attempted attacks.
Police said two barrels, each containing 22lbs of explosive, washed up on Israel's shores yesterday. A bomb squad defused one, the second blew up as a bomb disposal robot checked it. No one was hurt.
By today police were sweeping the entire Israeli coast in jeeps and on horseback. They also shut down a 25-mile beach strip near Gaza.
A spokesman for the Popular Resistance Committees, a Gaza faction with close ties to Hamas, said the attack was meant to avenge the killing of Mahmoud al-Mabhouh in Dubai.
Hamas and its Iranian patrons have accused Israeli agents of killing al-Mabhouh, a veteran Hamas operative who Israeli defence officials say was involved in smuggling rockets into Gaza.
Israel has refused to comment on the allegations.
"We confirm that there are still many of these charges in the ocean, and they haven't exploded yet. The Zionist enemy should expect more of these operations from the hands of the Palestinian resistance," the spokesman said.
Hamas has dramatically scaled back its own operations against Israel since a devastating Israeli military offensive a year ago, apparently reluctant to provoke further action. Smaller groups now typically claim responsibility for any attacks.
"The attack was an intended terrorist attack that failed," a police spokesman said.
"Police assessed the security situation throughout last night and continue to keep the beaches closed."
Sailors and fishermen were warned of possible dangers in the waters. In Ashdod, a port city where one of the barrels washed up, a crane-like robot crawled along the beach, prodding suspicious objects.