WASHINGTON, Jan 18: Iraqi guerillas have used increasingly sophisticated weapons to attack US aircraft, senior army officials told The New York Times on Sunday.

A classified army study found that the guerillas have used rocket-propelled grenades, as well as more advanced heat-seeking surface-to-air missiles to attack military helicopters, senior officials familiar with the study told the paper.

The study was conducted before the latest three attacks this month on US military helicopters, but those incidents only strengthened the study's findings, they said.

The study made recommendations on how to help pilots evade such weapons, but the officials declined to discuss those findings. The latest attacks also raised fears that the guerillas were carefully studying flight patterns of US aircraft.

"It's hard to say whether it's been a bad couple of weeks or it's something larger. But clearly, that area has us concerned," one senior officer in Baghdad told the paper.

Army officials were particularly concerned by findings that on at least one occasion, the guerillas used an SA-16 shoulder-fired missile, which has an advanced guidance system that is difficult for pilots to evade.

"The enemy has clearly seen the possibilities from earlier successes," said one senior Army aviator in the Persian Gulf region. "The enemy enjoys a strategic success each time one of our aircraft is shot down. It becomes a major media event, and questions arise as to who is winning. So the enemy sees this as very useful."

Nine military helicopters have been shot down or crash landed since October 25, killing 49 soldiers. US military helicopters are equipped with jamming devices, chaff dispensers, and sensors capable of detecting when their aircraft has been illuminated with a hostile missile targeting radar. But pilots have to rely on other tactics and their reflexes to fend off rocket-propelled grenade and small arms fire attacks.-AFP