Hanoi  - Two Vietnamese Air Force pilots were killed when their fighter jet crashed into a hill in northern Vietnam, a military source said Friday.

An official at Yen Bai military airfield, who declined to be named, said the Russian-built MIG-21 fighter crashed during a routine exercise Thursday in the mountainous province of Thanh Hoa.

The accident killed Lieutenant Colonel Nguyen Van Vinh, head of Air Force Regiment C31, and another pilot. Both pilots' bodies were recovered from the wreckage.

The news website VnExpress quoted Tran Cong Thanh, chairman of the Yen Bai People's Committee, as saying the fighter had crashed into a warehouse near a residential area.

Hoang Quoc Cuong, deputy chairman of Yen Bai People's Committee, said the fighter had encountered trouble while flying over a residential area. Cuong said the pilots had guided it out of the residential area rather than ejecting.

Nguyen Van Hung, a resident of the area, said he saw the plane spin several times, discharging black smoke, and explode.

The cause of the accident was under investigation.

The Vietnamese Air Force has been flying the MIG-21 since the mid-1960s, and still has hundreds in operation. The common model is a single-seat fighter, but Vietnam also flies a two-seat training configuration. (dpa)