Former army chief General Jehangir Karamat will be Pakistan's ambassador to the US.
He will take over from Ashraf Jehangir Qazi, who was appointed UN Secretary General Kofi Annan's special envoy to Iraq.
The US has approved the appointment of Gen Karamat, who was army chief from 1996 to 1998, sources said.
Gen Karamat, who had quit his post as army chief following differences with the then prime minister Nawaz Sharief, was succeeded by Gen Pervez Musharraf.
Musharraf took over the power from Sharief in a bloodless coup a year later in 1999.
Gen Karamat was active in the Track-II diplomacy between India and Pakistan and has been serving as a visiting fellow at Stanford University and the Washington-based Brookings Institution.
He was also part of a UN-sponsored study on Afghanistan and is the chairman of Islamabad Policy Research Institute's board of governors.
Gen Karamat is now in New York attending the UN General Assembly session as part of President Musharraf's delegation. Qazi had resigned in July following his appointment as the UN envoy to Iraq.
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