WASHINGTON, July 17 (Reuters) - U.S. Defense Secretary Robert Gates will ask Congress for approval to shift $1.5 billion from some military programs to the purchase of specially designed armored vehicles to protect troops in Iraq, defense officials said on Tuesday.

The Pentagon chief has made the purchase of Mine Resistant Ambush Protected vehicles, or MRAPs, one of the Defense Department's top acquisition priorities.

The vehicles' V-shaped hull is designed to protect occupants from roadside bombs, which have proven deadly to U.S. troops in both Iraq and Afghanistan.

Gates will call some members of Congress on Tuesday evening, one defense official said.

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