It was the third time Israel had staged retaliatory air strikes in Gaza in the past fortnight, with each raid following rocket fire on the Israeli south, raising fears of a new confrontation with Palestinian militants.

Aircraft hit "terrorist infrastructure" in northern Gaza, a military statement said.

The Israeli government also ordered the closure until further notice of the Erez crossing for people and the Kerem Shalom crossing for goods.

Only medical cases and humanitarian aid will be allowed through, it said.

Palestinian security sources and witnesses said the air force targeted a training site belonging to Hamas's armed wing, the Ezzedine al-Qassam Brigades, in the northern Beit Lahiya area.

The rocket fire on Saturday evening struck open ground near the southern Israeli port of Ashkelon, causing no casualties. The same area was targeted by three rockets on Wednesday night, which also prompted retaliatory air strikes.

After that attack, the military deployed batteries of its Iron Dome air defense system around Ashkelon as a precaution, Israeli media reported on Friday.

The latest attack was claimed by a relatively-new group of Islamic extremists called the Supporters of the Islamic State in Jerusalem, which has claimed responsibility for all three instances of rocket fire in the past two weeks.

The first attack was on May 26.

The group is locked in power struggle with Hamas which is the de facto power in Gaza, with the Islamist movement's security forces mounting a harsh crackdown on such fringe groups.

Last week, Hamas police shot dead a local jihadist leader during an arrest operation in Gaza City.

Since last summer, when Israel and Hamas fought a deadly 50-day war in and around Gaza, there have been growing signs of internal unrest in the territory with a spate of bomb attacks targeting public buildings and officials as well as international organizations.

Although few have been claimed, they are believed to be the work of radical Salafists unafraid to challenge Hamas.

Salafists are Sunni Muslims who promote a strict lifestyle based on that of early "pious ancestors". In Gaza, they have made no secret of their disdain for Hamas over its observance of a tacit ceasefire with Israel and its failure to implement Islamic law.(++++)

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