Singaporeans who have visited Iran, Iraq, Sudan or Syria for leisure in the past five years must now obtain a visa to enter the United States, under a law passed there in the wake of last year's Paris attacks.
Travel agencies in Singapore expect the new rule to affect only a small group of Singaporeans, as most people who go to the four countries do so for business.
A spokesman for travel agency Chan Brothers Travel said: "We do not anticipate any immediate impact as the changes to traveller eligibility will only affect a small group of Singaporeans."
Singapore is one of 38 countries in the US visa waiver programme.
Citizens of these countries who were previously able to travel to the US for up to 90 days without a visa must now obtain one if they have visited Iran, Iraq, Sudan or Syria since March 1, 2011, the US State Department said yesterday.
They must also obtain a visa if they are dual Iranian, Iraqi, Sudanese or Syrian nationals.
Several of the Islamic State in Iraq and Syria attackers who killed 130 people in France held European passports that would have allowed them to enter the US under the former system.
Citizens of the 38 countries are required to obtain a travel authorisation through the Electronic System for Travel Authorisation (ESTA) before going to the US.
The US State Department said that as of yesterday it would revoke the ESTA travel authorisations held by citizens from the 38 countries if they are dual Iranian, Iraqi, Sudanese or Syrian citizens.
The US Secretary of Homeland Security may waive these restrictions on law enforcement or national security grounds.
People who may be eligible for a waiver include those who visited the countries on behalf of international organisations or humanitarian groups, or journalists who reported there.