WASHINGTON — Homeland Security Secretary Jeh Johnson acknowledged concerns Friday that terrorists might use the visa waiver program to enter the United States, and said his department is taking steps to address weaknesses in the program.

Johnson told an aviation industry luncheon that he doesn’t want to discard the program, which makes it easier for Americans to travel to friendly countries and for citizens of those countries to travel to the U.S. “It represents an important element of lawful commerce between and among our international partners,” he said.

But he noted that some of those countries also have citizens or legal residents who have left to fight or train with terrorist groups in the Middle East, Asia or Africa, then returned home intent on violence. For example, some citizens or residents of France and Germany have traveled to the Middle East to fight or train with al-Qaida or Islamic State militants. The concern is that those fighters will return to their home countries and from there travel to the U.S.

In November, the department added new information fields to the electronic system for travel authorization, Johnson said. He said he has also asked his staff to tighten the security assurances the U.S. has with countries that participate in the waiver program.

“To deal with the foreign fighter potential, the foreign fighter threat presented now globally, we need to develop more robust information sharing with our key counterterrorism allies overseas to share information about individuals of suspicion,” Johnson said. “There is much work to be done.”

Foreign fighters already known to the U.S. are less likely to enter the U.S. without a visa than those unknown to intelligence agencies. For example, the two brothers French authorities said conducted the deadly attack on the Charlie Hebdo newspaper in Paris – Said Kouachi, 34, and his brother Cherif, 32 – were already on the U.S. no-fly list. Said Kouchaci had traveled to Yemen and Cherif Kouachi had served 18 months in prison for recruiting militants to fight the U.S. in Iraq.

Johnson also said the 15 Customs and Border Protection clearance centers established at overseas airports to screen airline passengers bound for the U.S. have been successful.