A U.S. warplane came under anti-aircraft artillery fire while flying over northern Iraq but was not hit, U.S. officials said Wednesday.

It was the first time U.S. military officials had disclosed an attack on a U.S. aircraft since allied troops crossed into Iraq last month to aid Kurdish refugees.But later Wednesday, a Defense Department official, speaking on condition of anonymity, said there had been four such incidents previously over northern Iraq.

The official, who was in Bahrain with Defense Secretary Dick Cheney, said the earlier attacks were not discussed with Iraqi authorities because the source of the anti-aircraft fire was not known. It was not known who was responsible for Tuesday night's attack, either.

Allied troops Wednesday awaited the go-ahead to move into this provincial capital, in what would be a major expansion of the allied miltary effort in northern Iraq. In southern Iraq, by contrast, the last U.S. troops pulled out.

President Bush, in Washington, said the latest attack was being investigated.

The president also told reporters that "I don't want to see us get into a quagmire" in northern Iraq, but said: "What we're doing is humanitarian."

The U.S.-led occupation of the area is aimed at reassuring the Kurdish refugees who fled after Iraqi troops in March crushed their rebellion following the Persian Gulf war.

Thousands of Kurdish refugees were heading home Wednesday by trucks, buses and farm wagons, leaving behind spartan border camps and the graves of many who did not survive the ordeal of their trek into the mountains.

Last month, Iraqi President Saddam Hussein and Kurdish leaders reached an agreement in principle on a plan to grant more Kurdish autonomy. The official Iraqi News Agency said Saddam met Wednesday with Kurdish leader Massoud Barzani, but gave no details about their discussion.

INA said deputy Prime Minister Tariq Aziz and representatives of three other Kurdish rebel factions were also present.

Meanwhile, the fate of 5,000 refugees displaced by the failed uprising in southern Iraq is still uncertain. A U.N. fact-finding team was in Kuwait to try to determine their needs.

Caring for the refugees in the south had been one of the principal tasks of U.S. troops after the war's end. In Wednesday's pullout, elements of the 3rd Armored Division crossed over into northern Kuwait as the last of the American forces withdrew.

At Incirlik air base in southern Turkey, a U.S. spokesman said it was not immediately clear who shot at the A-6 Navy bomber over northern Iraq.

The plane was not damaged and the pilot was not injured, according to a statement from Incirlik, headquarters of the allied relief operation for the Kurdish refugees.

U.S. aircraft continued Wednesday to fly over Dohuk, 25 miles south of the Turkish border. Allied troops on Sunday reached the outskirts of Dohuk but had not yet moved into the city. Military officials were waiting Wednesday for a decision from their superiors.

Allied commanders have acknowledged that if their mission of getting the Kurds home is to succeed, they must expand the security zone to include Dohuk.

Iraqi troops also appeared to be setting up machine guns and sandbags about two miles from U.S. units near the city, but U.S. military officials have reported no problems with the Iraqis.

At least 335,000 Iraqis, most Kurds, escaped to the border between Turkey and Iraq, where they faced disease, starvation and cold. An estimated 1.5 million Kurds fled to the Iranian border area to the east, where efforts to return them to Iraq have moved slower.

In other developments:

- Cheney was scheduled Wednesday to go to Oman, Qatar and Bahrain, presenting Bush administration proposals for expanding the American military presence in the region. The proposals include storing military equipment and supplies on the Arabian Peninsula, increasing joint U.S.-Arab military exercises and setting up a small permanent Army headquarters in the area.

- Iraq Wednesday released three CBS technicians who were missing after passing the last American checkpoint on the road to Basra, Iraqi government officials said.

The three who vanished after heading toward the southern Iraqi city were identified as Bud Mills, 56, and Barry Severson, 26, both satellite technicians from the United States, and Richard Parrott, a videotape editor from Britain.

- Canada said it has granted permanent resident status to Mohammed al-Mashat, who served as Iraq's U.S. ambassador before the war. He left Washington shortly before the war began Jan. 17 and filed immigration papers in Vienna, Austria.