WASHINGTON — The Obama administration on Thursday announced changes to a visa-waiver program that would make it harder for travelers to enter the United States from Europe if they had dual citizenship from Iran, Iraq, Sudan or Syria, or had visited one of those countries in the last five years.

Those travelers will now have to go through the more rigorous regular visa application process to enter the country. The Department of Homeland Security, which announced the changes, said they would take place immediately.

The administration’s plan would provide limited exemptions for individuals who have to travel to any of the four countries as diplomats or for military service. Additional exemptions could be applied for humanitarian reasons or for journalists.

About 38 countries, mostly in Europe, participate in the visa-waiver program, which allows their citizens to visit the United States without a visa on trips of 90 days or less. About 20 million tourists use the program each year.