Israeli forces shot dead four Palestinian "commandos" at sea off Gaza today, medical sources and witnesses said, as the military said it had foiled a "terror attack".

Palestinian witnesses said they saw Israeli helicopters and naval forces firing on a vessel off the coast of Nusseirat, south of Gaza City, at around 4 am.

Two hours later, four bodies in diving suits were pulled from the water and taken to hospital, medical sources and witnesses said, describing the dead as "commandos".

Searches were under way for another two people still believed to be missing after the pre-dawn attack.

The Israeli military said a naval force in the area had attacked a boat carrying "a squad of terrorists wearing diving suits on their way to execute a terror attack".

"The force fired and hit the terrorists," a military spokesman said, without giving any further details of the operation.

Several hours later, there was an Israeli air strike in northern Gaza, which injured an armed Hamas militant, Palestinian medical and security sources said.

The strike occurred just east of Jabaliya, they added.