ISLAMABAD - Twenty-five Pakistani security forces have been killed in fighting in northwest Pakistan after about 200 militants crossed over from Afghanistan and attacked a security checkpost, a government official said on Thursday.

Skirmishes had been going on for 24 hours in a village in Dir region, Ghulam Mohammad Khan, a top local government official, said by telephone.

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"We are preparing to evacuate 25 bodies from the village," he said, referring to police and paramilitary forces killed in the fighting that spread to a forest. Khan said he had no figures on militant casualties.

Dir police official Dawar Ali said army troops had arrived in the area to support security forces. He said more details on the battles were hard to come by because communications had broken down in the area.

The militants had arrived in a pre-dawn operation on Wednesday dressed in military uniforms and killed one policeman.

Pakistan's Taliban movement, which has close ties to al-Qaida, has stepped up pressure on the US-backed government after vowing to avenge the killing of Osama bin Laden by US special forces on May 2 in a Pakistani town.

It has stepped up suicide bombings, attacking paramilitary cadets, a naval base, a US consulate convoy and other targets.

After the bin Laden raid, the United States reiterated its call for Pakistan to crack down harder on militancy, especially on groups that cross over to Afghanistan to attack Western forces.

It's not clear which militants had taken on security forces in Dir, but groups along the frontier are closely linked.