An American carrying a sword and pistol who told police he was on a mission to kill Osama bin Laden has been arrested in a remote mountain forest in northern Pakistan.

Police said they detained Gary Brooks Faulkner, a construction worker from California, as he attempted to cross the border with Afghanistan in Chitral district.

"He told the investigating officer he was going to Afghanistan to get Osama. At first we thought he was mentally deranged," said Muhammad Jaffar Khan, the Chitral police chief.

But when police realised he was carrying a loaded pistol, a 40in sword and night-vision goggles, Khan said, "we realised he was serious".

Faulkner's age and background were not clear. Khan said he was born in 1969 but some reports put his age at 52.

A US embassy spokesman confirmed that a US citizen had been taken into custody and would be visited by a consular official. He declined to comment further, citing privacy laws.

Faulkner arrived in Chitral on 2 June and travelled to the Kalash Valley, a remote enclave of non-Muslim tribal people that adjoins the Afghan province of Nuristan.

The American checked into a small hotel in Bumburet Valley and was assigned a police escort. Local authorities have been nervous since a Greek development worker, Athanasius Lerounis, was kidnapped by Taliban militants last September.

Lerounis, who was held in Afghanistan, was released unharmed after eight months. But the American was actively seeking Islamist militants.

He went missing on Sunday night and slipped away from his guard and headed towards an exclusion area around the Afghan border, which is unmarked and largely unmanned.

"We had to stop him for his own safety. Foreigners are not allowed to visit that area," said Khan, the police chief.

He initially resisted arrest, threatening to fire on police, and later told interrogators he was going to Nuristan "to decapitate Osama bin Laden", Dawn newspaper reported.

Khan, the police chief, said Faulkner had visited Pakistan seven times previously, three of them to Chitral and always as a tourist. He had purchased the pistol during a previous trip. "He must have hidden it somewhere," he said.

The American was also carrying a book containing Christian verses and teachings. Asked how he thought he would track Bin Laden in such a remote area, he told one officer: "God is with me, and I am confident I will be successful in killing him."

Bin Laden watchers have occasionally identified Chitral, a quiet mountainous district at the northern end of Khyber Pakhtunkhwa (formerly North-West Frontier Province), as a possible hiding place for the al-Qaida leader.

Chitral covers a sprawl of sweeping, snow-sprinkled peaks that lead towards K2, one of the world's most dangerous mountains. But locals say it would be virtually impossible for the Saudi militant to remain undetected in Chitral because extremist militants have little support in the area.

Faulkner has been taken to Peshawar for questioning by Pakistani intelligence. If the police do not press charges he is likely to be deported.

A western diplomat in Islamabad described the incident as "another wacky story from the frontier".