Two ultralight aircraft collided near an aerodrome in northeastern Spain on Sunday, killing four passengers, regional authorities said.

Firefighters found one plane on fire in a wooded area near the Moia aerodrome north of Barcelona after being alerted by a witness who saw it lose altitude and crash, the regional government of Catalonia said.

"Once the fire was extinguished, firefighters found two lifeless bodies inside the ultralight (plane)," it said in a statement.

Several hours later firefighters found a second damaged plane with two dead people inside about 300 metres (984 feet) from the first.

The authorities believe the two aircraft collided in the air.

Police and civil aviation authorities have opened an investigation into the accident.