An RAF Tornado F3 crashed during a routine training flight in Argyll, Scotland at about 11.45 BST.
The aircraft was based at the RAF Leuchars in Fife, the UK Ministry of Defence (MoD) said.
It is not known whether the pilot and navigator were able to eject safely but no civilians are thought to have been hurt in the crash, the BBC reports.
Strathclyde Fire and Rescue said its officers were helping in the rescue operation.
"Strathclyde Fire and Rescue personnel are working alongside personnel from the RAF, the police and ambulance services to deal with this incident and to locate the occupants of the aircraft," a statement said.
The MoD said the circumstances were being investigated.