A passenger plane bound for Thailand from Singapore was escorted back to its departure airport by two fighter jets after a bomb threat by a passenger that turned out to be a false alarm.

After searching the Scoot jet - a low-cost airline owned by Singapore Airlines - and the baggage of the person that made the threat and his two travelling companions, police said no such threat was found.

Three passengers were assisting police with their investigations, they said.

Scoot said all 173 passengers and six crew aboard the Airbus A320 were safe and that flight TR634 would resume its journey to Hat Yai later on Thursday evening.

Singapore has a near-perfect record of keeping the city state free of terror, but it has markedly stepped up efforts to deter militancy in recent years after Isis militants briefly took over a town in the southern Philippines last year.

Singapore's defence minister said in a Facebook post that two F-15SG fighter jets had immediately responded to the threat when alerted by the pilot and escorted the plane from the South China Sea back to Changi airport.

"For our RSAF pilots who are on stand-by duties 24/7, every threat is considered real until proven otherwise," said Ng Eng Hen.

A Scoot staff member at the airline's check-in desk at Changi told Reuters that the incident had caused some delays to flights which had since been cleared up.