Four Palestinian fighters have been killed in an Israeli attack in northern Gaza, local medics and the Israeli military have said.

"There was an aerial attack against a [rocket] launching squad in the northern Gaza Strip," an Israeli military spokesman said.

Medics from Shifa hospital said they had received the bodies of four members of the Popular Resistance Committees.

The raid came shortly after two rockets hit uninhabited areas of Israel without causing any casualties or damage, the Israeli army spokesman said.

Tensions in the region have flared up in recent weeks as the five-month ceasefire continues to unravel.

Jacky Rowland, Al Jazeera's correspondent in Jerusalem, said: "The ceasefire doesn't seem to be anything more than a name at this stage."

She said the Israeli defence ministry had "made it clear if there are rocket attacks, they will attack the rocket launchers".

'No land incursion'

Senior Israeli defence sources told our correspondent there were no plans as yet for a large scale land incursion but that the army would continue to attack fighters firing rockets into the country.

Israel has stepped up its blockade of the Strip by keeping border crossings closed so preventing the delivery of fuel and essential humantarian supplies, while Palestinian fighters have launched scores of rockets into southern Israel.

The Israelis have almost continuously enforced the blockade since November 5, halting the supply of United Nations (UN) food and medical aid to 750,000 Palestinians and forcing the territory's sole power plant to shut down.

Peter Lerner, Israel's liaison officer for the Palestinian territories, confirmed the border crossings remained closed.

"Consultations on their eventual reopening will take place during the day," he said.

International organisations and human rights groups have urged Israel to ease the blockade.

UN Secretary General Ban Ki-moon "is deeply concerned at the deterioration of the humanitarian and security situation in Gaza and southern Israel. He calls on all parties to uphold international humanitarian and human rights law," a UN statement said.