In a statement provided to The Washington Post, Department of Homeland Security Secretary Jeh Johnson, shown here in September, said that the changes are needed “to learn more about travelers from countries from whom we do not require a visa.” (Manuel Balce Ceneta/AP)

The United States has imposed tighter screening measures on travelers from European countries and other allied nations amid concern over the rising number of Islamist militants who have fought in Syria and hold Western passports, U.S. officials said.

The heightened security steps were scheduled to be introduced Monday by the Department of Homeland Security and are aimed at dozens of countries whose ­citizens are exempted from requirements that they obtain visas before entering the United States.

The new rules will require millions of travelers from Europe, Australia and elsewhere to provide more detailed information to U.S. authorities before they can board a flight to the United States.

For the first time, U.S. officials said, such travelers will be required to disclose whether they hold passports from multiple countries, or have previously used alternate names or aliases — data that can enable more accurate screening against U.S. terrorism watch lists.

Homeland Security Secretary Jeh Johnson said in a statement provided to The Washington Post that the changes are needed “to learn more about travelers from countries from whom we do not require a visa.” He said the new requirements “will not hinder lawful trade and travel.”

U.S. officials said the changes were driven by concern over how many citizens from “visa-waiver” countries have fought in Syria and, because of their citizenship, hold passports that enable them to travel relatively freely across Europe and potentially to the United States. Many have joined an affiliate of al-Qaeda or a rival group, the Islamic State, that has seized territory and executed American and British hostages.

“Many of the leading visa-
waiver countries are seeing their citizens going to Syria to join [the Islamic State] or al-Qaeda affiliates in that country, and potentially returning home with training and new skills,” a senior Department of Homeland Security official said. “We want to ensure that we know exactly who is coming and have the most information possible to make good decisions.”

U.S. officials have said that more than 15,000 foreign fighters have traveled to Syria. The vast majority are from countries in the Middle East or North Africa whose citizens need a visa to enter the United States. But more than 2,000 have come from Australia and countries in ­Europe.

The overall flow into Syria has remained steady at more than 1,000 a month despite hundreds of airstrikes in recent months and efforts by countries including France, Germany and Britain to prevent their citizens from departing for Syria’s civil war.

The new DHS measures are designed to augment a screening system that was launched in 2008 at the recommendation of the commission that investigated the Sept. 11, 2001, attacks.

Known as the Electronic ­System for Travel Authorization, the program requires travelers from most Western countries to submit detailed information online and obtain an authorization code before boarding an aircraft or ship to the United States.

The DHS official said that the system is being expanded to gather additional information about travelers’ employment and addresses as well as their aliases and passports.

The department has processed about 63 million applications since the program’s inception, the official said. Most are approved instantly. But about 22,500 cases required travelers to undergo additional screening by U.S. consular officials because of discrepancies in their passports, the official said. And about 4,300 have been denied permission to travel because they were linked to entries in the United States’ ­database of known or suspected terrorists.

The system is aimed at travelers who would ordinarily undergo only light scrutiny before coming to the United States. Travelers from most countries — including Syria, Yemen and Pakistan, where al-Qaeda affiliates or other Islamist militant groups are based — are required to apply in person for a visa and undergo interviews and background checks.