WASHINGTON, Oct 23: The Pentagon is reconsidering its policy towards Guantanamo Bay detainees after some of those released rejoined terrorist groups in Afghanistan and Pakistan, US officials said on Saturday.

Officials at the Pentagon say that at least eight former inmates had rejoined the groups attacking US, Afghan and Pakistani forces along the Pakistan-Afghan border.

These include Abdullah Mehsud whose group claimed responsibility for the recent kidnapping of two Chinese engineers in Pakistan.

The Pentagon officials say the discovery that some freed prisoners have rejoined terror groups can delay the release of other detainees who now may be detained for years. They say that plans are now under way for building an additional medium security prison at Guantanamo to house more detainees.

The Pentagon officially says that there were approximately 660 detainees, of whom 120 have been released, leaving 559 inmates at Guantanamo. But lawyers and human rights groups say the actual number could be higher, about 650 to 800.

There were an estimated 60 Pakistanis at Guantanamo Bay and according to the Pakistan Embassy in Washington all but three had been released.

US officials, under pressure from their own Supreme Court and international human rights groups for holding them for years without charge and, in many cases, without access to lawyers, began releasing the prisoners in late 2002.

All freed prisoners were first interrogated by a panel of US officials and released only after the interrogators became sure that they posed no future threat. But cases like Mehsud's highlight the difficulty in separating those who could be released from those believed to represent an on-going threat.

Mehsud told reporters last week that he successfully hid his identity - of a Taliban-linked Islamist militant - during two years in US custody. Mehsud was released in March after a three-year detention.

The so-called hawks within the US establishment have always argued that those being held at Guantanamo Bay have intelligence value in the war on terrorism.