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 on Tuesday.

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 all to enforce the 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 out of control from the sky.

The no-fly zone is intended to protect civilians from attack by forces loyal to Muamar Gaddafi. The uprising against Gaddafi's four-decade rule began February 15.