Palestinian militants said Tuesday they had launched a large number of floating explosives into the Mediterranean Sea to avenge the death of a top Hamas commander, a day after two bomb-laden barrels washed up on Israel's coastline.
Israeli authorities launched an intensive search for new bombs, closing miles of beaches, deploying robotic bomb squads and ordering surfers out of the wintry waters.
Israeli Prime Minister Benjamin Netanyahu blamed Hamas and hinted at retaliation.
Early Wednesday, Israeli warplanes struck two smuggling tunnels under the Gaza-Egypt border in response to the barrel bombs and the firing of a rocket from Gaza, the military said. Such airstrikes are automatic reactions to militant attacks.
Abu Saed, a spokesman for the Popular Resistance Committees, a Gaza faction with close ties to Hamas, said the attack was meant to avenge the killing last month of Mahmoud al-Mabhouh, a veteran Hamas operative who Israeli defense officials say was involved in smuggling rockets into Gaza.
Hamas says Israeli Mossad agents ambushed al-Mabhouh in his Dubai hotel room, immobilized him with an electrical shock and strangled him to death.
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," Abu Saed said, standing alongside five other gunmen in military fatigues.
A joint statement from his group and two other small factions — Islamic Jihad and the al-Aqsa Martyrs' Brigades — said they sent eight bombs and detonated them by remote control. Hamas did not sign the statement.
In Jerusalem, Netanyahu accused Hamas, which rules Gaza, of being behind the operation, with support from Iran and Syria.
"We view with great severity the Hamas operation near the Gaza beach, and we will respond accordingly," he said at a news conference with visiting Italian Prime Minister Silvio Berlusconi. He did not elaborate.
On Monday afternoon, a barrel bomb washed up on the beach of Ashkelon, about nine miles north of Gaza. A few hours later, another one was found at Ashdod, six miles farther north.
Each had about 22 pounds of explosives, police said. They said bombs of that size could cripple small civilian vessels but not Israeli warships.
The barrels should not pose a threat to shipping lanes in the east Mediterranean since the tides would just carry them back to shore.
The second barrel blew up as a police robot was examining it, sending the tractor-like device tumbling through the air, according to witnesses. A police bomb squad defused the other one. No one was hurt.
Maritime authorities warned sailors and fishermen to be alert to possible dangers in the waters. In Ashdod, a crane-like robot crawled along the beach, prodding suspicious objects.
The military linked the barrels to two explosions at sea on Friday, aimed at Israeli ships. Israeli analysts speculated that the beach barrels might have been meant for ships but floated ashore instead.
"The attack was an intended terrorist attack that failed," police spokesman Micky Rosenfeld said. He did not refer to the Hamas threat of retaliation.
On Tuesday, Iran joined Hamas in publicly accusing Israel of carrying out the al-Mabhouh assassination, calling it "yet another example of state-sponsored Israeli terrorism."
Israel, the EU and the United States shun Hamas as a terror group. Hamas overran Gaza in 2007, expelling forces loyal to Western-backed Palestinian President Mahmoud Abbas, who now controls only the West Bank.
In response, Israel clamped a blockade on Gaza, allowing only essential humanitarian supplies to enter.
Last winter, Israel launched a massive three-week offensive in Gaza to try to stop years of near-daily rocket barrages by Palestinian militants, killing about 1,400 Palestinians, many of them civilians.
Since then, rocket fire has dropped considerably, and Hamas has been seen to keep an informal cease-fire, though other groups have attempted to carry out attacks against Israel.
A rocket exploded in Israel on Tuesday, the military said, causing no damage. A previously unknown Palestinian militant group claimed responsibility on an Islamic Web site.