WASHINGTON-- CIA Director Mike Pompeo said Thursday there was nothing "untoward" about his meeting with top Russian spy chiefs, saying that even though Moscow remains an adversary, ignoring chances to cooperate on security issues would endanger American lives.

Pompeo sent a letter on Thursday to Senate Minority Leader Chuck Schumer, who had raised questions about the meeting. Schumer said it was suspicious because it came just days before the Trump administration decided not to issue new sanctions at this time against Russian politicians and oligarchs over Russian interference in the election.

Pompeo told Schumer that U.S. intelligence officials meet periodically with their Russian counterparts to discuss various topics, including counterterrorism, aviation security and preventing foreign fighters from returning to both nations.

Pompeo did not identify the Russians he met with in Washington last week, but two U.S. officials identified them as Sergei Naryshkin, head of the Russian Foreign Intelligence Service, and Alexander Bortnikov, who directs the top KGB successor agency known as the Federal Security Service. The officials were not authorized to disclose the information and spoke on condition of anonymity.

The two Russian intelligence officials were sanctioned in 2014 in response to Russian aggression in eastern Ukraine -- Naryshkin by the U.S. and Bortnikov by the European Union.

Schumer said sanctions against Naryshkin impose severe financial penalties and prohibit his entry into the U.S. without a waiver.

In his letter, Pompeo said the meetings with the Russians were not without discord.

"When those meetings take place, you and the American people should rest assured that we cover very difficult subjects in which American and Russian interests do not align," Pompeo wrote.

"Neither side is bashful about raising concerns relating to our intelligence relationships and the interests of our respective nations. We vigorously defend America in these encounters and pull no punches -- we never will."

The CIA said any interactions with foreign intelligence officials would have been conducted in accordance with U.S. law and in consultation with appropriate departments and agencies.

Government agencies at times need to secure waivers to get certain individuals into the country, according to a U.S. official, who agreed to discuss the process only on condition of anonymity. The official said law enforcement agencies, for instance, might need a waiver to allow a witness in an arms trafficking case to travel to the United States. Other U.S. agencies might need to speak with officials from an enemy nation.

Still, Schumer wants to know why the visit was allowed, who approved it, which other Russian officials were in the delegation and whether they also are under sanctions. Schumer wrote his own letter on Thursday to National Intelligence Director Dan Coats, calling on him to disclose what was discussed at the meetings and whether the Russians also met with Trump administration officials at the White House, State or Defense departments or the National Security Council.

"Did the U.S. officials who met with Mr. Naryshkin raise Russia's interference in the 2016 elections? If not, why was this not raised? If raised, what was his response?" Schumer asked.