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Both men suffered bullet wounds during an attempt by Gaza security forces to arrest two men from a Salafist group, followers of an ultra-conservative form of Islam, a witness said.

The young man was in serious condition, while the policeman's condition was not life threatening, the medical source said.

The witness said a grenade was thrown at security forces raiding a house in Al-Fukhari in the southern Gaza Strip, sparking clashes.

Hamas, which rules Gaza, is an Islamist party but is frequently criticised by more conservative Islamists, including hardliners who sympathise with the Islamic State group.

The hardliners sporadically fire rockets at Israel, prompting retaliation against Hamas targets.

Israel holds Hamas responsible for all rocket fire from Gaza, regardless of who launches it.

In an unrelated incident, a fighter of Hamas' military wing, the Al-Qassam Brigades, was killed accidentally.

"Ahmad Mansour, 30 and from Jabalia in northern Gaza, died on Monday night when a grenade exploded by accident," a Qassam statement said.