WASHINGTON, May 22 (Reuters) - President George W. Bush has chosen Anne Patterson to be ambassador to Pakistan, which the United States views as a key ally in fighting terrorism, the White House said on Tuesday.

If confirmed by the U.S. Senate, Patterson would replace Ryan Crocker, who earlier this year became U.S. ambassador to Iraq.

Patterson has had extensive exposure to the region in her current job heading the State Department's bureau for international narcotics and law enforcement affairs, which oversees some U.S. programs to train Afghan police and which seeks to combat the booming narcotics trade in Afghanistan.

One of Patterson's top challenges would be getting the Pakistani government to do more to prevent Taliban and al Qaeda forces from crossing its long, porous border into Afghanistan.

Patterson previously served as deputy U.S. ambassador to the United Nations and U.S. ambassador to Colombia.

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