All passengers and crew on Scoot Flight TR632 disembarked safely.

The passenger suspected of making the threat and two travel companions were assisting police investigations, police said.

Scoot is the budget carrier of Singapore Airlines.

Two F-15SG fighter jets responded to the pilot’s alert and escorted the plane from the South China Sea back to Changi airport, said Singapore Defence Minister Ng Eng Hen in a posting on Facebook.

“For our RSAF pilots who are on stand-by duties 24/7, every threat is considered real until proven otherwise,” said Ng in a Facebook post.

No further details were immediately available.

Singapore has a near-perfect record of keeping its shores free of terror, but it has markedly stepped up efforts to deter militancy in recent years with more frequent attacks on Western countries and after Islamic State militants briefly took over a town in the southern Philippines last year.

Flight Aware plane tracker said Flight TR634 was a twin-engine Airbus A320.

Scoot said it was “working closely with the authorities for necessary follow-up to ensure the safety of our guests”.

Changi Airport Group said its ground resources were on standby and Airport Police were currently investigating.