FBI Director Jim Comey testified in a Senate hearing Wednesday that he has “concerns about the ability to vet people” from countries with which the U.S. does not have a relationship, such as Syria and Yemen.

Comey was asked by Republican Louisiana Sen. John Kennedy, a member of the Senate Judiciary Committee, whether it is possible to “properly vet” a non-citizen from a “conflict area such as Syria.”

“It’s difficult to do it perfectly, and I have concerns about the ability to vet people coming from areas where we have no relationship on the ground with the government there,” Comey replied. “I suppose it’s possible to do it reasonably. There are a number of tools you could bring to bear, but there are always risks associated with that.”

The FBI director said these “tools” are querying the holdings of the U.S. intelligence community to find matches for an individual’s contact information, like phone numbers, email and home addresses. He said that in Iraq, the U.S. relies heavily on biometric information obtained by military forces there. Sen. Kennedy asked about vetting people from Yemen, and Comey responded that it is “similarly difficult.”

President Donald Trump signed two executive orders temporarily banning the immigration of individuals from countries with which the U.S. has a weak relationship. The first one included Yemen, Iran, Iraq, Somalia, Syria, Sudan, and Libya. The second one did not bar the entry of Iraqis. Both have been held up in federal courts and 1,493 refugees from Yemen and Syria have resettled in the U.S. since Trump’s inauguration.