The deadly incident comes a week after nine pro-Palestinian activists were killed in a clash with naval commandos as their vessel tried to reach Gaza. The interception prompted a worldwide outcry and growing demands that Israel ease or lift its blockade of Gaza.
Israel has been enforcing a blockade of Gaza since 2007, when the Islamic militant Hamas seized the territory. The closure also includes a maritime blockade that was highlighted in recent days when Gaza-bound aid ships were intercepted by the Israeli navy.
In Monday's incident, a naval force spotted Palestinians in diving suits in the waters off Gaza and opened fire, the military said. It said its forces prevented an attack on Israeli targets, but did not provide any further details.
In a text message sent to reporters in Gaza, Al-Aqsa Martyrs' Brigades — the remnants of a violent offshoot of Palestinian President Mahmoud Abbas' Fatah faction — said the four men killed were members of its marine unit who were training in Gaza's sea.
The message said more details would be available at a press conference later Monday.
Four bodies were retrieved and taken to a hospital in central Gaza, said Moawiya Hassanain, a Palestinian health official. Two of the dead had suffered multiple gunshots to the head.
The Palestinian naval police said two people were still missing.
Also Monday, Palestinian officials said Israel fired a missile at Palestinian militants near the Gaza border, wounding one.
