Changi Airport has stepped up security checks since Malaysia Airlines Flight MH370 went missing more than two weeks ago.

In a joint statement on Monday, the Singapore Police Force and Immigration and Checkpoints Authority said that the security alert has been raised "as a precautionary measure".

No details were given but The Straits Times understands that since the incident, more strict checks are being done for selected flights. Airlines at Changi Airport have also been briefed to stay vigilant.

As the search continues for the missing Malaysia Airlines plane, it was confirmed that at least two passengers had boarded the aircraft, which was en route from Kuala Lumpur to Beijing, with stolen passports.

At Changi Airport, in addition to security procedures and policies, the authorities also work closely with the airport community including cleaners and restaurant staff who help play the role of eyes and ears on the ground, the statement said.

"The police and ICA are monitoring the security threat situation closely. We appreciate the understanding and patience from travellers as we conduct the various security checks to ensure safe and secure travel."