Immigration agents arrested six Russian nationals with suspected ties to the ISIS terror group following a cross-country sting operation, according to a report Tuesday.

The report followed a recent series of warnings to American lawmakers by FBI Director Christopher Wray about increasing concern regarding "the potential for a coordinated attack here in the homeland, akin to the ISIS-K attack we saw at the Russia concert hall in March."

Immigration and Customs Enforcement officials reportedly revealed the arrests to the New York Post, which said the arrests followed an investigation prompted by the FBI.

One suspect was allegedly overheard talking about bombs on a wiretapped phone, and a source told the Post that it raised the specter that something like the 2013 Boston Marathon bombing "might happen again or worse."

Those arrested are reportedly originally from Tajikistan, a former Soviet republic that Foreign Affairs magazine last month described as a new, fertile recruiting ground for the Islamic State Khorasan, also known as ISIS-K.

The U.S. has said intelligence confirmed the group's claim of responsibility for a March 20 shooting and arson attack that killed at least 144 people in a concert hall in suburban Moscow.

The arrests reported Tuesday reportedly took place over the past week in New York, Los Angeles and Philadelphia.

None of those arrested were identified and the charges against them were unclear.

Neither ICE nor the FBI immediately returned requests for comment, the Post said.