(Feb 9): US aviation investigators are probing an incident in which a private plane and President Donald Trump’s aircraft flew closer than was permitted, three people familiar with the event said.

The two aircraft got to about two nautical miles from each other over Florida on Feb 3. Planes under the supervision of air-traffic controllers are supposed to stay at least three nautical miles from each other near airports and as far as five nautical miles apart at higher altitudes.

The people, who asked not to be named because they weren’t authorized to talk about the case, said that there was no risk of collision as the planes were flying on parallel courses.

When the president is flying around the country, a number of special safety and security provisions are enacted. FAA air-traffic supervisors pay closer attention and Secret Service officials also monitor the airspace for possible threats. In many cases, other flights are halted or diverted to create extra space around Air Force One.

Trump flew to Palm Beach International Airport on Friday, arriving at about 4.30pm local time. The incident occurred about 48.2km from the airport, one person said.

A so-called "loss of separation" can be caused by a controller error or a pilot mistake. All turbine-power aircraft, including the Boeing Co 747 that normally carries the president, are equipped with devices that track other aircraft and issue warnings to prevent mid-air collisions.

The Federal Aviation Administration, which declined to comment in an e-mailed statement, is investigating. The National Transportation Safety Board, which has authority to examine aviation incidents, has also been notified.