The Pentagon is more than doubling the number fighter jets it is sending to a NATO air policing mission in the Baltics and is sending one aerial refuelling aircraft, a US official said, in what is part of Washington's response to the Ukraine crisis.

Defense Secretary Chuck Hagel announced plans to bolster the decade-old NATO mission, which patrols skies over Baltic allies Estonia, Latvia and Lithuania.

A U.S. official, speaking on condition of anonymity, said the Pentagon will send six additional F-15 jets and one KC-135 refuelling aircraft to augment the mission this week.

The United States already provides four F-15 jets to the mission, which helps identify and respond to violations of Baltic airspace.

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