Hamas militants had shortly before taken over a key headquarters of the Fatah-affiliated National Security Forces in the northern Gaza Strip after several hours of fighting that left 21 people dead, witnesses and security sources said.
Both Hamas and the security forces confirmed that the compound had been totally occupied by Hamas militants.
After the Hamas raid, the Central Committee of the Fatah movement convened in Ramallah and decided to suspend the participation of its ministers in the unity government until the fighting ends, but held back from pulling its ministers out of the coalition completely.
The joint Fatah-Hamas coalition was agreed in March following a summit in Mecca, in an attempt to end an earlier cycle of internecine violence.
Earlier in the day Hamas' armed wing, the al-Qassam Brigades, declared the northern Gaza Strip a "closed military zone" and issued an ultimatum that all members of the Fatah-dominated security forces leave their headquarters "within two hours from now."
Fatah officials accused Hamas of assuming the status of a properly constituted "army" and called the actions an attempt to wage a "coup" against institutions affiliated with President Abbas, a member of Fatah.
Seven people were reported killed in various other incidents between Fatah and Hamas gunmen during the day, bringing the death toll in the past two days of intense infighting to 43 people, said Mo'aweya Hassanein, from the Palestinian Ministry of Health.
Meanwhile, Israeli Prime Minister Ehud Olmert, meeting with Dutch Foreign Minister Maxime Verhagen, said that the latest violence was a worrying sign, according to a statement issued by his office.
"If the Gaza Strip ultimately falls to Hamas, this will be of great regional significance," he said.
Olmert said that the West should seriously consider placing a multi-national force on Gaza's southern border, to prevent the strengthening of extremist elements.
Palestinian Prime Minister Ismail Haniya, from the Islamic Hamas movement, issued a statement Tuesday night in which he called for "an end to the fighting, and to resume negotiations between Fatah and Hamas."
In the statement he accused the "secular Fatah party led by Mahmoud Abbas" of not supporting "political partnership."
Abbas earlier issued a desperate plea to battling rival Palestinian factions, calling for an immediate truce in the Gaza Strip to end the bloody internecine clashes in the salient.
"Abbas calls on everyone to cease the infighting and to immediately initiate a dialogue. He also ... urges everyone to place the higher national interests over narrow personal interests and preserve the Palestinian blood, history and future," a presidential statement said.
The statement accused "a small group of leaders," including those unhappy with a national unity deal between Fatah and the ruling Hamas of "taking the country into an ugly civil war."
Fatah's armed wing, the al-Aqsa Martyr's Brigades, threatened for its part to expand the fighting to the West Bank by announcing it would kill Hamas officials there unless the Islamic organization ceased its attacks.
On Tuesday morning, alleged Fatah militants in the southern Gaza town of Khan Younis abducted and killed the nephew of Abdel Aziz Ranteesi, a senior leader of the governing Hamas movement who was assassinated by Israel in April 2003.
Two more people - both affiliated with Fatah - were killed later in the morning, and four others were killed in the afternoon.
Hamas militants, meanwhile, took over all police and security installations in Khan Younis, and approximately 90 officers in forces affiliated with Fatah laid down their arms and surrendered, eyewitnesses and officials said.
Earlier Tuesday, hundreds of Hamas militants surrounded the Gaza house of Fatah spokesman Maher Megdad shortly after he appeared on local radio and television strongly condemning what he called the "inhuman war" waged by Hamas against Fatah and the killing of a senior Fatah official and his brother in an attack the previous night.
In the early morning, suspected Fatah gunmen fired a mortar shell at the Gaza City house of Prime Minister Haniya. The shell hit the upper floor, causing damage but no injuries, witnesses said.
Haniya was believed not to have been at home at the time. On Monday, the premier had halted a meeting of his cabinet due to exchanges of fire taking place near his Gaza City office.
Hamas gunmen retaliated by launching four mortar shells near the presidential compound of Abbas.
Attempts by Egyptian mediators to reach a new ceasefire seemed blocked, when leaders of both groups said they were unable to discuss a truce, because they could not reach the office of Egyptian General Burhan Hamad due to roadblocks set up throughout Gaza City by both sides. dpa sar mak jab ok sg ls wjh ds cc
- Deutsche Presse Agentur
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