Gunmen from the radical Islamist movement ambushed two seats of the Fatah loyalist national security -- the main Palestinian security force -- in Gaza City and Jabaliya provoking clashes with those holed up inside.

Security officials and witnesses at the scene reported heavy fighting but were unable to give immediate reports of any casualties.

The attack came after mortar shells slammed into prime minister Ismail Haniya's home and the seafront compound of president Mahmud Abbas in the latest bout of fighting that has killed 18 people in 24 hours.

The apparently no-holds barred conflict threatens the very foundations of a Hamas-Fatah coalition that took office less than three months ago in a bid to halt the feuding that has killed nearly 180 people since December.

Abbas's office called for an immediate ceasefire but accused leaders in Hamas of plotting a coup and leading Gaza toward civil war while his secular Fatah faction warned it could pull out of the shaky, Hamas-led government.

"All the information and all the facts point to a faction, to which political and military leaders of Hamas belong, who are plotting a coup against Palestinian legitimacy," the presidency said.

It charged that the Hamas leaders in question were "pushing the homeland towards the throes of a dreadful civil war" and issued a plea on behalf of Abbas for an immediate ceasefire and serious dialogue.

Fatah announced that the party's central committee was examining whether it should remain in government and slammed Hamas for "looking to annihilate the Palestinian Authority and create a republic of hate and death" in Gaza.

Haniya's office weighed in with a statement charging that "parties linked to enemies tried to bring down the national unity government militarily."

Defying the latest Egyptian-brokered ceasefire, Gaza plunged into renewed violence five days ago just weeks after a bout of strife left 54 people killed when tempers boiled over how to implement a flagship government security plan.

Palestinian civilians cowered in their homes as shooting and explosions reverberated through the streets outside.

"Since early morning we have heard explosions and shooting. We can't go and buy any food, we can't stand in front of the windows, all our children are in one bedroom. The situation is very bad," said Adnan, a doctor in Gaza City.

A national security officer was killed in the central Gaza Strip on Tuesday after Hamas fighters encircled two camps of the Fatah-loyalist force, bringing to at least 18 the number of Palestinians killed since Monday.

And for the first time since the latest violence began, troubles spread to the occupied West Bank where Hamas official and Palestinian under secretary for transport Saidi Tamimi was snatched by gunmen who stormed into his department.

Also in Ramallah, the presidential guard said it raided an office of Hamas television station Al-Aqsa and confiscated equipment. Three employees were arrested, said security sources, sparking swift Hamas condemnation.

Overnight, assailants fired more than 50 mortar shells against the Gaza City headquarters of the preventive security service loyal to Fatah and activists torched around a dozen homes of Fatah and Hamas members, security sources said.

Hamas activists also kidnapped two Palestine TV technicians and blew up the building where they were abducted in Gaza City, a television official said.

Twenty-four Palestinians have now been killed in the lawless and radicalised Gaza Strip, awash with weapons, since the latest bout of internecine bloodshed erupted last Thursday following weeks of calm.

An International Committee of the Red Cross official said four patients were killed and 10 people wounded on Monday at Beit Hanun hospital, which was no longer functioning and nearly occupied by one party to the conflict.

"We cannot concentrate or write answers because of the sound of bullets of explosives. We're frightened," said Islam, a student sitting his school matriculation exams in Tal al-Hawa where fighting had been among the heaviest.