A US Marine transport helicopter crashed in flames in a field northwest of Baghdad, killing all seven people aboard, the US military said. It was the fifth US aircraft lost in less than three weeks and the latest sign of growing problems with aviation in Iraq. A US military statement gave no reason for the crash of the CH-46 Sea Knight, which went down Wednesday near Fallujah in Anbar province, about 30 kilometers from Baghdad. However, at the Pentagon, three Marine Corps officials said the troop-transport helicopter was in flames when it went down, with the pilot appearing to attempt a hasty landing but losing control as the aircraft descended. They said witnesses in nearby Marine aircraft saw the flames but saw no sign that it involved hostile fire. An Iraqi air force officer, however, said the helicopter was downed by an anti-aircraft missile. An Iraqi farmer who lives about a kilometer (half mile) from the crash site said he heard a missile fired moments before the crash, which took place in an insurgent-infested region. "The helicopter was flying and passed over us, then we heard the firing of a missile," the farmer, Mohammed al-Janabi, said. "The helicopter then turned into a ball of fire. It flew in a circle twice and then went down." Associated Press Television video showed the flaming wreckage lying in a field in front of a cluster of mud homes. A dense plume of black smoke rose over the remains. The Marine officials suspected the fire was caused by a mechanical problem, the officials said. They spoke on condition of anonymity because the investigation was ongoing. Names of the victims were not released, but military officials said they included five Marines and two from the Navy. In a statement posted on an extremist Web site, an al-Qaida-linked group, the Islamic State in Iraq, claimed it shot down the helicopter, which it described as a Chinook - an Army helicopter which resembles a Sea Knight. Critics have long urged the military to replace the CH-46, which was introduced in 1964 at the start of the Vietnam War. In 2001, retired Col. Frank Jensen wrote in Defense News that the Marines should replace the CH-46 but cannot because of budget limitations. Regardless of the cause, the latest crash adds urgency to a US military review of flight operations in Iraq, including whether insurgents have perfected skills in attacking US planes. The latest crash occurred five days after a US Army Apache helicopter went down in a hail of gunfire north of Baghdad. Three other helicopters - two from the Army and one operated by an American security firm - also have crashed since Jan. 20. A total of 27 people - 23 US service members and four American civilian contractors - were killed in those five crashes. The military did not say whether all seven killed in Wednesday's crash were Americans, but it was likely that they were. The first four aircraft that crashed were all believed to have been shot down, the military has said, raising new questions about whether Iraqi extremists are using more sophisticated weapons or whether US tactics need changing. Any fresh threat to aviation would present serious problems for US commanders as they launch the new security crackdown in Baghdad. The US military relies heavily on helicopters in Iraq, not only for supporting ground forces in combat but also to move troops and equipment by air to avoid roadside bombs and insurgent ambushes. At night, US attack helicopters prowl the darkened skies over Baghdad and other cities, using night-vision equipment to hunt for insurgents and militiamen planting roadside bombs or setting up firing positions. Gen. Peter Pace, chairman of the Joint Chiefs of Staff, has acknowledged that insurgent ground fire in Iraq has been increasingly effective. "I do not know whether or not it is the law of averages that caught up with us or if there's been a change in tactics, techniques and procedures on the part of the enemy," Pace told a Senate committee Tuesday. In December, a spokesman for Saddam Hussein's ousted Baath party, Khudair al-Murshidi, told The Associated Press in Damascus, Syria, that Sunni insurgents had received new stocks of shoulder-fired anti-aircraft missiles and "we are going to surprise them." However, Pace said Tuesday that the four helicopters that crashed since Jan. 20 were brought down by small arms fire rather than missiles. On Wednesday, the Stars and Stripes newspaper quoted a US officer as saying insurgents brought down an Apache helicopter last week near Taji by concentrating heavy automatic fire on the aircraft - rather than by using a shoulder-fired missile. Helicopter transmission gears and rear rotor assemblies are vulnerable to ground fire and cannot be protected by armor plating because of the weight. Sea Knight helicopters, with their distinctive dual rotors, are used by the Marines primarily as a cargo and troop transport, and can carry 25 combat-loaded troops, according to the think tank GlobalSecurity.org.