Baghdad, Iraq ? A U.S. Army helicopter crashed Friday in a hail of gunfire north of Baghdad, police and witnesses said – the fourth lost in Iraq in the last two weeks. The U.S. command said two crew members were killed, and the top U.S. general conceded that insurgent ground fire has become more effective.

An al-Qaida-affiliated group claimed responsibility and said its fighters had “new ways” to attack American planes.

A brief U.S. military statement gave no reason for the crash and did not identify the type of aircraft. A Pentagon official said it was an Apache attack helicopter, which carries two crew members.

The official spoke on condition of anonymity because he was not authorized to release the information. Another Apache crashed Sunday during heavy fighting with a Shiite cult near Najaf, also killing two soldiers.

Iraqi police and witnesses said the latest crash occurred about 7:30 a.m. as two Apaches were flying along a well-established air route near Taji, a major U.S. base about 12 miles north of Baghdad.

One helicopter was struck by heavy machine gunfire but continued flying, the witnesses said. The other helicopter banked sharply and flew back toward the source of fire, apparently to attack the target.

But that helicopter was also struck by ground fire, exploded in a ball of fire and crashed, the witnesses said. The other helicopter flew away, they said. The witnesses spoke on condition of anonymity, fearing for their own safety.

The United States has lost more than 50 helicopters in Iraq since May 2003, about half of them to hostile fire.

However, the loss of four helicopters since Jan. 20 has raised questions about whether insurgents are using more sophisticated weapons or whether U.S. tactics need changing.

Three of the latest crashes involved Army helicopters – two Apaches and one Black Hawk. The fourth was an OH-6A observation helicopter operated by the Blackwater USA security firm. All were believed shot down, and 20 Americans, including four civilian, died in the crashes.