Israeli warplanes launched fresh raids in the Gaza Strip on Sunday, after earlier attacks killed nine Islamic Jihad militants and retaliatory rocket fire killed one Israeli.

The first Israeli attack early Saturday afternoon killed five members of the Al-Quds Brigades, Islamic Jihad's armed wing, said Adham Abu Selmiya, spokesman for Gaza's emergency services.

Another three militants were critically wounded, he added.

As tit-for-tat fighting continued into the night, Israeli aircraft struck more targets in Gaza, witnesses and Palestinian officials said.

Four more militants were subsequently killed and at least two wounded -- at least two of them as they tried to fire a Grad rocket into Israel, an Al-Quds spokesman said.

The two latest raids Sunday targeted Zeitoun, east of Gaza City, but no one was hurt in the attack, said witnesses.

As rockets and mortar shells pounded Israel Saturday, Israel's police said they were raising their national alert level to its second-highest.

The Israeli military said the Rafah raid had "targeted a terrorist squad in the southern Gaza Strip responsible for the firing of military-use projectiles towards the Israeli home front."

The other strikes were aimed at similar targets, it said.

The military said of the earlier raid that the air force had fired on a "group of terrorists preparing to fire long-range rockets" and that the attack had "prevented the attempted firing."

It said the men had also been responsible for firing a Grad rocket into Israel on Wednesday that hit near the city of Ashdod, 35 kilometres (22 miles) from the Gaza border.

The strikes were the bloodiest since a tacit ceasefire was agreed between Gaza Palestinian militants and Israel in late August.

Reprisal attacks began after sunset, and police said that by mid-evening 21 rockets had been fired from Gaza into southern Israel.

Two people were initially reported wounded and two others slightly injured, but one of them, injured by shrapnel, later died in hospital, Israeli medical officials and police said.

One rocket slammed into a community centre and another into a block of flats, setting parked cars and gas canisters alight.

The city of Ashdod, the nearby town of Gan Yavneh and the city of Ashkelon, to the south, were all hit, police said.

Other rockets hit open ground elsewhere in southern Israel and one was fired towards the city of Beersheeva in the Negev desert but appeared to have struck open ground, police said.

A statement released by Israeli Prime Minister Benjamin Netanyahu's office said he had phoned the mayors of Ashkelin, Ashdod and Beersheeva.

He had told them that the military's "tough response will be even tougher if necessary", said the statement.

A spokesman for UN Middle East envoy Robert Serry called for restraint.

"The recent escalations are very worrying. It's vital to de-escalate now, without any delay," spokesman Richard Miron said in a statement.

"We strongly appeal for calm and an end to violence and bloodshed."

The Al-Quds Brigades, in its own statement, claimed responsibility for the rocket fire, posting a video on its website that it said showed the launching of five of the the rockets.

Spokesman Abu Ahmed accused Israel of carrying out its raid in order to heighten tensions so it could renege on freeing 550 Palestinian prisoners.

Their release was agreed as part of a prisoner-swap deal with Gaza rulers Hamas for the liberation of Israeli soldier Gilad Shalit.

Israel released 477 prisoners in exchange for Shalit earlier this month and is due to free another 550 within two months.

A spokesman for Hamas's Ezzedine al-Qassam Brigades also blamed Israel.

"The occupation is completely responsible for the crime in Rafah and all of the resistance factions cannot leave the shedding of our martyrs' blood unanswered," spokesman Abu Obeida said.

"We shall discuss the answer to this crime."

The Israeli air force carried out three raids on the Gaza Strip Thursday in retaliation for the earlier attack, its planes hitting a base of the Ezzedine al-Qassam Brigades, witnesses said.

An Israeli army spokesman said of those strikes that aircraft had "attacked three terrorist sites in the Gaza Strip as well as an arms factory in the south of the territory."