ANCHORAGE, Alaska - The National Transportation Safety Board is investigating how a US Airways passenger jet flew within a third of a mile of a cargo plane at Anchorage's airport after the airliner's pilots refused to make a turn that they said would "put their flight in direct conflict" with the departing cargo plane.

The near-collision occurred about 12:10 a.m. May 21, when US Airways Flight 140, an Airbus A319 arriving from Phoenix with 138 people aboard, missed an approach and was turning to make a second landing try, the NTSB said Friday. It came within an estimated 100 feet vertically and one-third of a mile horizontally of the Cargolux Boeing 747-400, which was bound for Chicago.

"It's a very, very serious loss of separation in controlled airspace," Federal Aviation Administration spokesman Mike Fergus said Friday. He said two possibilities were pilot or controller error.

The southern end of a runway at Ted Stevens Anchorage International Airport lies near the western end of another runway. The US Airways jet was trying to land from the north when its pilots began a missed approach procedure because of tail winds, the NTSB said. The tower controller told the crew to turn right and watch for the Cargolux plane taking off to the west, then instructed the US Airways jet to continue to turn and climb to 3,000 feet.

"The A319 crew refused the right turn because the turn would have put their flight in direct conflict with the B747," the NTSB said. The US Airways crew then complied with a command to descend and was clear of the conflict at an altitude of about 1,700 feet.

Officials at the U.S. office of Luxembourg-based Cargolux could not immediately be reached. Morgan Durrant, a spokesman for US Airways Group Inc., said the carrier was cooperating with the NTSB.