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Senior defense officials told NBC News that five Free Syrian forces, trained and equipped by the U.S. military, have been captured by al Nusra fighters in northeast Syria.

The capture raises the number of Free Syrian rebels that have been killed, captured or wounded by al Nusra in the past week to 14, out of a total of 54 that have been trained by the U.S. military.

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The five were captured within the past couple of days, the officials said. The circumstances were not clear, but officials added there was no opportunity for U.S. warplanes or armed drones to intervene. The U.S. military has not indicated it is preparing to launch a rescue mission for the five.

This is the third assault in a week against U.S.-friendly forces by al Nusra, which has pledged to kill any Free Syrian rebels trained and equipped by the U.S.

Early last week, al Nusra captured five commanders from the 30th Free Syrian Army division. On Friday, al Nusra killed one U.S.-trained Syrian rebel and wounded eight others before U.S. warplanes launched airstrikes, killing an unknown number of al Nusra and driving away the rest of the assault force.

Al Nusra captured the five Free Syrian forces on Monday. There was no U.S. military response.

RELATED: U.S. Drones Defending Syrian Rebels from Al Qaeda-Affiliated Militants

The U.S. is currently flying armed but unmanned drones over Syria out of Incirlik, the Turkish military airbase, to provide combat air support for the Free Syrian Forces. American warplanes are expected to begin flying missions out of Incirlik “within the next week or two” once the U.S. and Turkey sign the necessary agreements to lay down the ground rules and protections for American service members.

The U.S. is currently flying combat missions over Iraq and Syria out of Qatar, Jordan and off aircraft carriers in the Persian Gulf.