Joel Brenner said in a radio interview, "They are sending over an increasing and troubling number of intelligence officers into the United States," adding that Russia, China, Iran, Cuba were the most persistent and aggressive intelligence threats to the U.S.
Former head of FBI counterintelligence David Szady backed up Brenner's claims adding that Russian agents operated at the UN and embassies, and also arrived in the U.S. under the cover of students or businessmen.
Dmitry Simes, a political scientist with the Nixon Centre, believes the situation is having a negative effect on U.S.-Russia intelligence cooperation, citing a high-ranking intelligence source who said the U.S. was pretending it shared intelligence information with Russia, and Moscow pretended it used it in its work.
But Brenner disagreed with Simes stating there were lots of areas where Russia and the U.S. continued to work effectively, including intelligence.
The Russian side was not immediately available for comment.