A bomb threat that turned out to be a hoax forced a passenger plane en route from Warsaw to Egypt to make an emergency landing in Bulgaria on Thursday, the operator of the aircraft said.

The Airbus A320 landed just before dawn in the Black Sea city of Burgas, where authorities said all 161 passengers and crew were evacuated safely.

"A passenger informed the cabin crew about the explosive he might be carrying," Small Planet Airlines, which operated the flight, said in a statement. "(He) ... later admitted it was a joke."

Police were questioning the man, a 67-year-old Polish citizen, the carrier said.

Bulgarian special forces had found no explosives in an initial sweep of the plane and were conducting a second check, a police spokeswoman said, without confirming the threat had been a hoax.

Bulgaria had tightened controls at its borders and airports in recent days, she added, following Friday's militant attacks in Paris in which 129 people died.

She said a passenger who alerted crew about a possible bomb on board later admitted having consumed alcohol.

The plane was en route to the Red Sea resort of Hurghada. Small Planet Airlines said it was sending another aircraft to Burgas airport to enable the passengers to complete their journey. - Reuters