JERUSALEM - An unmanned Palestinian fishing boat laden with hundreds of kilograms of explosives blew up off the coast of Gaza on Monday in what the Israeli military said was an attempt to attack a naval patrol in the area.

There were no casualties, but the incident threatened to upset a relative calm in the volatile territory ruled by the militant group Hamas.

Israel maintains a tight naval presence around the Gaza Strip, part of a larger blockade of the area aimed at preventing arms from reaching Hamas.

Israel's military chief, Lt.-Gen. Gabi Ashkenazi, said the boat was spotted through heavy fog early in the day by a naval patrol. He said the crew monitored the boat and it exploded about 600 metres away from the naval vessel.

"Our estimate is that it was a few hundred kilograms of explosives," Ashkenazi said during a ceremony at military headquarters in Tel Aviv. "We estimate that it was an attempt to harm Israeli naval vessels that I'm glad did not succeed."

Military officials said they were still trying to determine how the explosives were detonated.

The army said the incident occurred off the coast of northern Gaza, near the Israeli border. The explosion was so powerful it could be heard in Gaza City, several kilometres to the south.

There was no immediate claim of responsibility.

In 2002, two Palestinians on an explosive-packed boat blew themselves up next to an Israeli patrol vessel, killing themselves and wounding four naval crewmen. Islamic Jihad, a small group backed by Syria and Iran, claimed responsibility at the time.

The Israel-Gaza border area has been tense but relatively quiet since Israel ended a three-week military offensive in January.