A private Austrian-registered jet carrying four people that was due to land in Germany but which flight controllers were unable to contact has crashed off Latvia's coast, Swedish authorities said.

Fighter jets from Germany, Denmark and Sweden were scrambled to try to make contact with the crew in the air as the plane continued to fly across northern Europe, "but they saw no one", Swedish search and rescue operation leader Lars Antonsson said.

The Cessna 551 aircraft, which took off from Jerez in Spain and had been due to land in Koln, flew over Swedish airspace in the Baltic Sea prior before crashing into the sea off Ventspils, Latvia, just before 6pm Irish time.

The plane flew relatively steadily until it neared the Latvian coast, when it rapidly lost altitude.

It crashed "when it ran out of fuel", Mr Antonsson said.

The nationalities of the four on board were not immediately known.

Latvian search and rescue teams were leading operations at the crash site and were being assisted by a Swedish coast guard plane and a rescue helicopter, he said, but "no human remains have been found".

It is not known what caused the plane to fly off course.

"We have no explanation at all, we can only speculate" about what happened "but they were clearly incapacitated on board", Mr Antonsson said.

Latvian authorities have yet to comment on the crash.