BRUSSELS — The U.S. wants foreign nations to “take responsibility” for their citizens who were captured while fighting for the Islamic State group in Syria, but will defer to those countries on how to handle the issue, U.S. Defense Secretary Jim Mattis said Tuesday.

“The important thing is that the countries of origin take responsibility for them. How they carry out that responsibility, there are a dozen different diplomatic legal or however ways,” Mattis told reporters traveling with him in Europe.

“I’m not a lawyer, I’m not an international law person. But I know one thing, which is they shouldn’t be let back on the street,” Mattis said. “Doing nothing is not an option.”

The topic was a major focus during a Tuesday meeting of the counter-ISIS campaign in Rome, but no solution appears to have been reached, with Mattis saying: “Right now, we do not have that [issue] fully resolved.”

There are hundreds of foreign nationals who joined ISIS only to be captured by the Syrian Democratic Forces in Syria. The question of what to do with them has become increasingly important as the SDF holding facilities begin to fill up. However, some nations are reluctant to accept those fighters back into their home countries or are unprepared to deal with them through the traditional legal system.

“It’s an international problem. It needs to be addressed,” Mattis said. “There is not a one way forward for all detainees right now. Some nations have specific interests where they want to bring people in; some nations do not have any detainees there, but they are equally worried.”

The secretary was noncommittal on whether sending detainees to Guantanamo Bay was an option under consideration, saying he didn’t want to “jump to offering solutions” before things had been worked out.

“I’m not willing to say anything on that right now,” he said.

However, he repeatedly stressed that the country of origin for these fighters has a “responsibility” for deciding what happens to them.

“The most important thing is we figure out how we’re going to deal with this so we can deal with it and we don’t paralyze ourselves and say: ‘Nothing we can do,’ “ he added.