Three people were killed after two small planes collided in mid-air in the US state of Colorado, authorities said.

The collision was first reported at 8:54 a.m. local time (1454 GMT) on Saturday, according to the Boulder County Sheriff's Office.

The US National Transportation Safety Board confirmed in a tweet that it is investigating the mid-air collision of a Cessna 172 and a Sonex Xenos aircraft near Longmont, Colorado.

First responders discovered two separate crash sites, Xinhua news agency reported quoting the Sheriff's Office.

"The first crashed aircraft was found in the 10,000 block of Niwot Rd on the south side. At this time there are two confirmed occupants in the aircraft, both of whom were deceased upon first responders arriving on scene," said the sheriff's office in a statement.

"The second crashed aircraft was found in the 9,700 block of Niwot Rd on the north side," and the one confirmed occupant in it was also deceased before responders' arrival, the sheriff's office said.

Officials said investigation of the incident is still in progress.