Singapore is one of 38 countries affected by changes to the United States visa-waiver programme, which allows visitors to enter the country without visas for visits lasting fewer than 90 days.

The changes are part of an effort by the US to guard against the perceived threat posed by people who have gone to the Middle East to join groups such as the Islamic State in Iraq and Syria.

In a release posted on the US Department of Homeland Security website, the department said that citizens of countries under the programme may fly to the Middle East to receive training, then enter the US to carry out attacks.

With the changes, countries will have to use computer programs and databases that automate the sharing of travel records and other information with the US.

These programs will also track lost and stolen passports. These countries will also be required to allow more US air marshals on flights to the US.

In addition, they will have to use passports that rely on biometric identifiers, such as fingerprints, and have electronic chips that contain a photograph of the holder.

However, as Singapore introduced biometric passports in 2006, Singaporean travellers are unlikely to be directly affected.

"As I have said a number of times now, the current global threat environment requires that we know more about those who travel to the United States," said Mr Jeh Johnson, secretary for homeland security, in a written statement. "This includes those from countries for which we do not require a visa."

The US Department of Homeland Security also plans to conduct a review of the countries participating in the waiver programme, to determine if they are following its requirements.

Travellers from countries that the US deems to have weaknesses in their screening procedures will likely be subject to additional scrutiny at US border checkpoints.

It is unclear when the changes will take place.

Linette Lai