A Fatah security official confirmed the building had been lost. At least 12 people were killed and 30 wounded in the fighting.

Earlier, Hamas fighters also overran several smaller Fatah positions in Gaza.

Hamas gunmen also exchanged fire with Fatah forces at the southern security headquarters in the town of Khan Younis, but did not launch a major assault there. The town's streets were empty as people huddled inside. One Hamas man was killed, according to Hamas and medical officials.

In Gaza City, Hamas fired mortars and explosives at the pro-Fatah Preventive Security headquarters, drawing return fire from watchtowers in the compound. Elsewhere, Fatah fighters killed four Hamas gunmen in a battle near the besieged house of a senior Fatah commander.

The State Department and the US Consulate in Jerusalem, warning of a "very dangerous security situation," advised journalists not to travel to Gaza and urged any there to leave.

Even before the current outbreak of violence, no Western correspondents were based in Gaza. As the violence escalated this week, most journalists were staying off the streets, covering the conflict from the windows of high-rise buildings and keeping in touch with their sources by telephone.

Hamas and Fatah have been at odds since the Hamas election victory ended four decades of Fatah rule. The sides agreed to share power in an uneasy coalition three months ago, but put off key disputes, including control over the security forces. Most of the forces are dominated by Fatah loyalists, while Hamas has formed its own militia and has thousands of gunmen at its command.

Beverley Milton-Edwards, a Hamas expert at Queens University in Belfast, Northern Ireland, said Gaza is heading for a final showdown. "This has become the existential battle for the soul of the Palestinian people," Milton-Edwards said.

Brutality has grown in recent days, with people shot at close range in street executions. On Sunday, a member of Abbas' presidential guard, Mohammed Sweirki of Fatah, was kidnapped and hurled off a 15-story apartment building, followed a few hours later by the killing of a Hamas fighter, Abu Kainas, thrown from the roof of a 12-story building in apparent retaliation. In all, more than 80 people have been killed since mid-May, most of them militants.

Human Rights Watch, blamed both sides. "Fatah and Hamas military forces have summarily executed captives, killed people not involved in hostilities, and engaged in gun battles with one another inside and near Palestinian hospitals," the New York-based group said in a statement.

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