The Pakistani government has delayed the visas of hundreds of U.S. officials and contractors, a move that has frustrated the State Department and could affect U.S. programs on the ground.

State Department Deputy Spokesman Robert Wood told reporters the reason for the delay was unclear, but it has prompted serious U.S. concerns, and the issue has been raised with "very senior levels" of the Pakistani government.

A senior State Department official who would not speak on the record because of the sensitivity of the diplomacy said Secretary of State Hillary Clinton has spoken with the Pakistani leadership about the visa delays affecting a variety of diplomats and contractors dealing with agriculture, security and military affairs.

"I can't give you any reason why they're being delayed," Wood said. "But we are working with our Pakistani counterparts to try to resolve these issues."

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