Israeli warplanes struck Gaza early Sunday for the second time in three days after cross-border rocket fire by an Islamic extremist group which is locked in a power struggle with Hamas.

It was the third time Israel had staged retaliatory air strikes on the war torn Gaza Strip in the past fortnight after three instances of rocket fire, all of which were claimed by Salafist extremists loosely allied with the Islamic State group.

During the raids, the air force hit "terrorist infrastructure" in northern Gaza, the military said. The government also ordered the closure until further notice of the Erez crossing for people and the Kerem Shalom crossing for goods.

Israel said it held Hamas responsible for all attacks emanating from the Palestinian enclave, where it is the de facto power, with Prime Minister Benjamin Netanyahu warning that nobody would prevent Israel from defending itself.

"I have not heard anyone in the international community condemn this firing; neither has UN said a word," he told ministers at the start of the weekly cabinet meeting.

"It will be interesting if this silence continues when we use our full strength to uphold our right to defend ourselves. Let it be clear: The spreading hypocrisy in the world will not tie our hands and prevent us from protecting Israel's citizens."

Palestinian security sources and witnesses said the raids targeted a training site belonging to Hamas' 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 port of Ashkelon, causing no casualties. The same area was targeted by three rockets on Wednesday, which also prompted retaliatory air strikes.

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

Since the end of a deadly 50-day war between Israel and Gaza militants last summer, there have been at least seven instances of rocket fire on southern Israel.

Four of them have taken place in the past six weeks, fuelling fears of a fresh confrontation in a territory which has lived through three wars in the past six years.

- Internal power struggle -

Since last summer, there have been growing signs of internal unrest in the territory, with Hamas engaged in a power struggle with smaller extremist groups.

There have been a spate of bomb attacks targeting public buildings and officials as well as international organisations. Although few have been claimed, they are believed to be the work of radical Salafists unafraid to challenge Hamas.

The last three attacks on Israel were claimed by a relatively new Salafist group called the Supporters of the Islamic State in Jerusalem.

It first emerged in July 2014, during the war with Israel, when it claimed rocket fire on the Jewish state.

There is very little official information on the size of the group but Palestinian sources believe it has dozens of members and hundreds more supporters.

Some of its leaders formerly belonged to Hamas' Qassam Brigades, while others used to be part of Islamic Jihad's Quds Brigades. They are understood to be in contact with other regional Salafist groups, notably in Syria and Egypt's Sinai.

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.

- Israel, Hamas as enemy -

Salafist groups have been angered by a Hamas crackdown, and claim around 100 of their members or supporters are currently behind bars.

Last week, Hamas police shot dead a local Salafist leader during an arrest operation in Gaza City. A day later, the Supporters of the Islamic State in Jerusalem claimed at least two rockets fired at Israel.

Salafist sources said indirect talks they had holding with Hamas over a possible prisoner release deal broke down on Saturday.

Several hours later, there was fresh rocket fire on southern Israel.

Claiming the attack, Supporters of the Islamic State said it was a "gift" to the prisoners held by both Israel and Hamas.

"This attack is a gift to our brothers and sisters in Jewish jails and our brothers in Hamas jails," it said, equating both Israel and Hamas as the enemy.