In a flexing of U.S. military muscle, Defense Secretary Chuck Hagel on Wednesday dispatched six F-15 aircraft to patrol the skies over Latvia, Estonia and Lithuania, more than doubling American warplanes' presence in the NATO mission.

Hagel said the move was done at the request of the three Baltic nations. His wording echoed Russian President Vladimir Putin's claim that Kremlin troops had seized effective control of the Ukraine's Crimean Peninsula at the request of deposed Ukrainian President Viktor Yanukovich.

Hagel also boosted U.S. training flights with Polish military airmen in a separate move intended to signal American resolve in the face of the recent Russian aggression.

But in congressional testimony, Hagel and Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, advocated caution and pushed a resolution of the nearly weeklong crisis that would avoid more direct military action.

"I urge continued restraint in the days ahead in order to preserve room for a diplomatic solution," Dempsey told the Senate Armed Services Committee.

The United States currently provides four F-15's to fulfill its part in NATO's "Baltic Air Policing" mission in which warplanes from 14 of the Western military alliance's member nations take turns patrolling the skies over the former Soviet republics.

Through April, the United States will add an additional six fighter jets to the mix, according to a defense official who requested anonymity in order to provide details of the beefed-up mission.

Pentagon officials are consulting with their Polish counterparts about increasing U.S. training flights there. Ten U.S. Air Force officers and other personnel now help train Polish military aviators in F-16 and C-130 aircraft.