LAS VEGAS (Reuters) - Former U.S. FBI Director James Comey said that social media companies needed to “worry” about foreign political propaganda on their networks, but he had few ideas on how to counter it.

FILE PHOTO: Former FBI director James Comey speaks about his book during an onstage interview with Axios Executive Editor Mike Allen at George Washington University in Washington, U.S. April 30, 2018. REUTERS/Jonathan Ernst/File Photo

In an interview with Reuters, Comey also said he would be leery of the Federal Bureau of Investigation trying to track propaganda in the United States, let alone take action against it, while acknowledging that it was a major problem for the U.S. political system.

“I don’t have a great answer for them,” Comey said of social media companies including Facebook and Twitter, which were major venues for what U.S. intelligence agencies have said was a Russian-sponsored effort to help President Donald Trump win the 2016 U.S. election.

Comey’s comments on Wednesday follow former Director of National Intelligence James Clapper’s conclusion in a new book that the Russian election meddling, which allegedly included illegal hacking and leaking of stolen information as well as propaganda, had a decisive influence in electing Trump.

Trump fired Comey as the FBI investigated the Russian election interference, setting the stage for the appointment of Special Counsel Robert Mueller and his wide ranging enquiries.

Comey has been criticized for the FBI’s failure to counter Russia’s election meddling while it was happening.

But Comey said the FBI should not get involved in fighting propaganda because it is a “rule-bound institution,” with strict policies that serve as an appropriate check on its power.

“You’d want to be very thoughtful about having the FBI, without having a predicated investigation, be monitoring speech in the U.S., because it’s often very difficult to tell, is it coming from a nation state?” Comey said. “So in theory that might involve collecting more broadly on speech in the United States.”

He said those same concerns had kept the FBI from tracking an influence campaign that included Russian-driven Facebook posts that reached more than 100 million people on that social network alone ahead of the 2016 election.

Comey avoided answering questions about the ongoing Mueller probe and his own role in the earlier version of the investigation, but he scoffed at Trump’s accusation this week that the FBI had planted a spy inside his 2016 campaign.

Comey said he could not comment directly on the claim, floated this week by Trump and Republican supporters in Congress.

More generally, Comey said, “The word `spy’ is not an accurate characterization in any case of the FBI’s use of confidential human sources, which are a critical tool in all of our investigations — people telling us things that they know.”

Asked whether he could deny that the FBI sent someone to get a job working full-time inside Trump’s presidential campaign, Comey laughed and said: “I’m tempted, but I’ve got to leave it to the Bureau to comment.”

Comey is best known in Silicon Valley for leading an Obama Administration charge against end-to-end encryption uncrackable by law enforcement.

In the interview, he conceded that one the technology companies’ major objections to giving U.S. authorities special access -- that it would then have to do the same for governments in Russia, China and elsewhere -- was “reasonable.”

But he said some companies were already aiding such regimes by storing data in those countries and allowing access to source code. If they were sufficiently worried, he said, they could stop doing business in those places.

Comey said his goal was a process under which companies would grant access to authorities only according to strict standards of due process, such as relying on independent judges. If the companies refused back-door access until the other countries changed their legal system, “it would be good for the people of China and Russia.”