Published: Mar 22, 2011 8:23 PM EDT
Updated: Mar 22, 2011 5:24 PM EDT

TRIPOLI, Libya- Heavy anti-aircraft fire is lighting up the skies over Tripoli and the sound of loud explosions is echoing through the Libyan capital after nightfall.

The source of the explosions was not immediately clear, but the gunfire appeared to signal a fourth night of U.S. and European air operations over Libya on Tuesday to enforce a no-fly zone.

In the previous night's operations, the coalition air campaign suffered its first loss with the crash of an American fighter jet in the rebel-held east. Both crew ejected safely as the aircraft spun from the sky.

The no-fly zone is intended to protect civilians from attack by forces loyal to Moammar Gadhafi in their battles with rebel fighters. The uprising against Gadhafi's four-decade rule began Feb. 15.

(Copyright 2011 by The Associated Press. All Rights Reserved.)