Israeli navy killed at least four Palestinians off Gaza coast early Monday, medical sources said.

The dead were members of Al-Aqsa martyrs brigades, an offshoot of Fatah party. As Israel says they were en route to carry out a terror attack, a spokesman for the Palestinian group said in a statement that they were learning diving.

Witnesses said that an Israeli gunboat fired intensively at a fishing boat off the coast and killed four people who were wearing diving suits.

The dead were apparently affiliated to a militant group, medics said.

Police from Hamas, the Islamic movement that controls Gaza, retrieved the bodies in the south of Gaza City and ambulances took them to Al-Aqsa hospital in Deir Al-Balah town.

An Israeli army spokesman said the naval forces thwarted a Palestinian attack. He provided no further details.

Hamas police is still searching for at least one missing diver.

Israel imposes a blockade on the Gaza Strip since Hamas took over the enclave in 2007. The closure is also applied on the sea.

Since the beginning of the second Palestinian uprising in 2000, armed Gaza-based groups have tried to attack Israeli naval vessels in Gaza waters or infiltrate Israel through the sea but the Israeli troops discovered the attempted attacks.

Meanwhile, a Palestinian militant was seriously injured when an Israeli unmanned place fired a rocket in northern Gaza Strip. The drone's attack was aimed at a group of fighters to the east of Jabalya refugee camp, according to witnesses.