Seven former CIA directors have asked US president Barack Obama to quash a criminal probe of harsh interrogations of terror suspects during the Bush administration. The CIA directors, who served both Democratic and Republican presidents and include three who worked under George W Bush, made their request in a letter this weekend to the White House.

Attorney general Eric Holder announced last month that he was appointing an independent counsel to investigate possible incidents of abuse by CIA personnel during interrogations. The incidents were referred by the CIA inspector general to the justice department during the Bush administration, but officials at the time prosecuted only one case.

"If criminal investigations closed by career prosecutors during one administration can so easily be reopened at the direction of political appointees in the next, declinations of prosecution will be rendered meaningless," wrote the former directors.

The Washington Post has reported that the justice department will focus on only two or three cases for possible indictment. One of them involved an Afghan prisoner who died after being beaten and chained on a cold night to a concrete floor without blankets.

In their letter, the former directors warned that the investigations could discourage CIA officers from doing the kind of aggressive intelligence work needed to counter terrorism.