Two US aircraft have been shot down over southern Iraq. A Black Hawk helicopter came under small arms fire, and an F/A-18 Hornet was hit by a surface-to-air missile, both near Karbala. It was the first fixed-wing aircraft to be brought down by enemy fire during the campaign. Further north, Republican Guard units moved early on Thursday to defend Baghdad from advancing US troops, US military sources say.

US ground forces advanced to within 32 kilometers (20 miles) of Baghdad on Wednesday in one of the most dramatic pushes of the war.

There are conflicting reports about the number of personnel aboard the helicopter when it crashed.

Initial reports said seven of the 11 people on board had been killed, while four wounded were rescued. But US Central Command says there were only six people on board.

Shortly afterwards, a single-seat F/A-18 Hornet fighter plane crashed over southern Iraq, Central Command told BBC News Online.

The fate of the F/A-18 pilot - who has not been named - is not yet known.

A statement from Central Command said the armed forces are "committed to accounting for all coalition personnel" - likely an indication that rescue operations are under way.

PHOTO CAPTION

A U.S. Navy (news - web sites) F-18 waits to get fuel from a KC 10 plane, unseen, over northern Iraq, Wednesday, April 2, 2003 while another refueling operation is seen in background. (AP Photo/Jockel Finck) - Apr 02 6:55 AM ET