So called enforced disappearance has long been a problem in strife-torn countries such as Nepal and Sri Lanka, but new patterns have emerged in South Asia related to the war on terrorism, such as the Pakistani cases, it said.

"Whilst many of those have eventually been acknowledged as being held in Guantanamo Bay, others are believed still to be held in Pakistani detention although their precise whereabouts remain unknown," the rights group said, referring to the US prison in Cuba.

Pakistan is a major ally in the US-led war on terrorism and has rounded up hundreds of suspected militants and many are believed to have been handed over to the United States.

"South Asia has a history of enforced disappearances, with tens of thousands of people going missing over past decades in countries such as Nepal and Sri Lanka," Amnesty International's deputy Asia director, Catherine Baber, said in a statement.

"It is very disappointing to see countries such as Pakistan join in the trend that one would hope would be declining," she said.

In Nepal, where Maoist rebels have battled government forces for the past decade, a government committee is investigating more than 600 cases of enforced disappearance, the rights group said.

Sri Lanka, where ethnic Tamil rebels began a war for a separate state in 1983, has one of the highest levels of unresolved enforced disappearance in the world, it said, though it did not give an estimate of the number of cases.

Up to 10,000 enforced disappearances have been reported in the Indian-controlled part of the disputed Kashmir region, where Muslim rebels have been battling security forces since 1989, it said.