Hamas militants opened fire on a jeep carrying officers of Palestinian President Mahmoud Abbas' Fatah-dominated National Security service in Gaza City on Tuesday, killing at least eight members of the force, witnesses said.
It was the worst single attack since deadly clashes between Hamas and Fatah resurfaced over the weekend for the first time since the rival groups formed a unity government two months ago with the main aim of ending Palestinian infighting.
In total, at least 11 Palestinians were killed on Tuesday in escalating violence in Gaza, bringing the death toll since the clashes erupted Sunday to at least 21.
The clashes have shattered the appearance of a conciliation between the two groups and threatened the future of the fragile unity government, which is already suffering from a continuing international boycott because of its platform's failure to meet international conditions for normal relations, including recognition of Israel and a renunciation of violence.
According to witnesses, Hamas militants shelled the National Security jeep, killing its eight occupants, as it was on its way from a checkpoint on the beach road in western Gaza City to a base at the Karni border crossing with Israel, to the east.
The gunmen had been lying in an ambush near the Karni crossing, they said.
At the same time, Hamas militants also fired mortar shells and rocket-propelled grenades at the Karni crossing and the nearby National Security base, drawing fire from soldiers on the Israeli side of the crossing in which at least one other Palestinian was killed.
An Israeli army spokeswoman said the soldiers opened fire when they saw two armed men running toward them and hit one. Witnesses said both were National Security officers who ran toward the Israeli side to escape the Hamas shelling.
The military spokeswoman said Israel closed the crossing as a result of the internal Palestinian fighting.
Elsewhere in Gaza City, gunmen later also ambushed another vehicle, of the Preventive Security force, killing Captain Yosef Mutair and injuring three other people.
The attack near Karni came hours after members of Abbas' presidential guard shot dead a Hamas commander, Ibraheem Menia, Tuesday morning when he refused to stop at a roadblock set up by them in eastern Gaza City.
Hamas' military wing, the al-Qassam Brigades, had vowed to avenge his death.
"We will deal with Menia's murderers in the same way we deal with the Israeli occupation's spies," it said in a statement.
Meanwhile, some 450 armed Fatah gunmen crossed into Gaza at the Rafah border crossing with Egypt.
The latest clashes have already sparked the resignation of one member of the unity cabinet, Interior Minister Hani al-Qawasmi, an independent but considered close to Hamas who said Tuesday he had no interest in the portfolio if Palestinian President Mahmoud and his Fatah security advisors did not give him free reign to deal with security.
Prime Minister Ismail Haniya, of Hamas, has taken on the interior ministry in his stead.
Appointing an independent to the portfolio was a cornerstone of the February 8 Mecca agreement that paved the way for the formation of the Hamas-Fatah unity government in March.
Finding a candidate accepted by both sides had been a main obstacle to reaching the agreement and al-Qawasmi's resignation could therefore lead a renewed dispute over who should hold the portfolio, which is crucial because it involves control over some security forces, while others are headed by the president.
The Mecca agreement was reached under Saudi-Arabian mediation after a months-long power struggle that evolved after Hamas beat Fatah in January 2005 legislative elections.