Army Times correspondent Sean D. Naylor, who is travelling with the 3rd Infantry, said the appearance of the aircraft raised fears of a chemical or biological attack although it was probably only a reconnaissance mission.

He said it was believed to be the first time enemy aircraft had flown over US ground forces since the Korean War in the early 1950s, and underlined the effectiveness of unconventional tactics against the US military.

Anti-aircraft gunners spotted at least two aircraft on Friday afternoon but were still waiting for higher command to issue the order to shoot their Stinger missiles when the planes disappeared over the horizon.

Ground vehicles and Apache helicopters were then dispatched to search for the aircraft but found no sign of them.

Staff Sergeant Billy Armstrong, an anti-aircraft squad leader, said he spotted one ultralight being steered by a single pilot about 900 feetmetres) above the ground to the east of his position.

"I was thinking, Oh shit! Its the first time in my 12 years that an air defense role is going to be played in a situation where we actually had the possibility of firing a Stinger missile," he was quoted as saying on the armytimes.com website.

Captain Ruel Smith, an anti-aircraft battery commander, said he was frustrated as he watched the plane fly out of sight.

"If I had authority to shoot it myself, we would have engaged it," he said, adding that he nonetheless understood the need for caution.

"A lot of it has to do with cluttered skies. There are a lot of friendly aircraft in these skies."

The prop-driven ultralights show up on radar screens much like helicopters, Smith said, explaining how the Iraqi pilots had managed to evade detection until they were within sight of the US troops.

"It moves slow, and it has a prop, so it looks to us like a helicopter and there are many, many, many helicopters here."

Intelligence officer Captain Jeff Ryals said around six ultralight planes had been spotted over US troops in the northern Kuwaiti desert before the war began on March 20.

He said that was a "test to see if they could violate our airspace without getting shot down.

"All the (Iraqi) special forces missions weve seen in the last 10 years have been strategic reconnaissance on motorcycles or infiltrators dressed as Bedouins.

"So it would be a new tactic if special forces used these (ultralights) for strategic reconnaissance."

He said the planes seen Friday were probably "scouting out the sexiest targets to strike with surface-to-surface missiles."

Smith said that in response to Friday's incident, air defence commanders now had the authority to shoot first and ask questions later if they fixed eyes on any more ultralights.

US commanders insisted Saturday the Iraqi air force was unable to fly a plane despite the sighting of the ultralights.

Major General Vistor Renuart told reporters here that the Iraqis did not have the capability to fly an airplane and had "not shown any inclination" to do so.

"We keep a very close eye on the Iraqi airfields. Weve kept them closed, we intend to continue to keep them closed," he said.

"Were concerned about any possible use of an airplane to conduct terror or military operations and we watch that very, very carefully."

WAR.WIRE