Two small planes have collided in mid-air near Denver, Colorado, killing three people, authorities in the US said.

It was not known what might have caused the crash between the four-seater Cessna 172 and the Sonex Xenos, a light, aluminium, homebuilt aircraft that can seat two.

Nearby residents who heard the crash and rushed outside told NBC affiliate 9News in Denver they were surprised the planes could have struck each other on such a clear morning.

A crashed plane, one of two, lies along Niwot Road between Highway 287 and N 95th Street in Longmont, Colorado
A crashed plane, one of two, lies along Niwot Road in Longmont, Colorado (Andy Cross/The Denver Post via AP)

One of the planes crashed in a field, the other in a stand of trees, near Vance Brand Airport in Longmont, Colorado, about 30 miles north of Denver.

Two people were found dead in one wreckage, and one person in the other, the Boulder County Sheriff’s Office said.

The Federal Aviation Administration and the National Transportation Safety Board (NTSB) are investigating.

The NTSB said a preliminary report would be released in about 15 days.

Emergency crews work at the scene of a plane crash involving two planes in Longmont, Colorado
Emergency crews work at the scene of a crash involving two planes in Longmont, Colorado (Andy Cross/The Denver Post via AP)

The collision happened just before 9am local time on Saturday.

The identities of the victims were not immediately released.