White House Warns U.S. Rivals Against Seeking Advantage

“I think our adversaries know that the United States government is steady at the tiller and that we’re protecting the American people,” he said on CBS’s Face the Nation Sunday.

In an interview with the Journal, Mr. O’Brien, who last week met with a top Russian official in Geneva, said U.S. alerts haven’t been raised and there was no expectation that rivals such as North Korea, Iran, China and Russia were likely to pose a new threat.

“Any attempt by an adversary to take advantage of the fact that the president has been diagnosed with Covid[-19] would be a mistake of serious magnitude,” he said Friday in the interview. “I don’t expect any country to make that mistake.”

Other officials said that there has been no known threat of significance made by adversaries since Mr. Trump became ill. Mr. Trump tested positive for coronavirus on Thursday and was hospitalized Friday at the Walter Reed National Military Medical Center in Bethesda, Md.

Mr. Trump was briefed on national security matters Sunday by Mr. O’Brien, Secretary of State Mike Pompeo and Army Gen. Mark Milley, chairman of the Joint Chiefs of Staff. Defense Secretary Mark Esper is traveling in the Middle East. The briefing, which was conducted by video teleconference, was considered routine and not in response to any particular threat, officials said.

“We know there’re rogue actors, we know there’re malign actors around the world,” Mr. Pompeo said, before departing on a trip to Japan, adding the U.S. is fully prepared for any challenge.

Mr. O’Brien met late last week with Nikolai Patrushev, the secretary of the Security Council of the Russian Federation, in Geneva before returning to the U.S. a day early after Mr. Trump’s diagnosis of Covid-19. They discussed bilateral issues ranging from Russian operations in Syria and Afghanistan to nuclear negotiations and the conflict between Azerbaijan and Armenia.

Mr. O’Brien said one of the most prominent messages he conveyed was that Russia shouldn’t meddle in the U.S. election next month. He told them that the U.S. was especially concerned about any efforts that would affect the vote on election day.

The Russians stated publicly they wouldn’t interfere. Mr. O’Brien said he would take a “trust-but-verify” approach to the Russians’ claims.

U.S. intelligence agencies have assessed that Russia interfered with U.S. elections in 2016 and is attempting to do so again this year, along with hackers associated with China and Iran. Russia, like the other countries, always has denied such interference.

State Department officials didn’t respond to a request for comment on steps the U.S. has taken to apprise foreign countries of the status of the U.S. government in light of Mr. Trump’s hospitalization.

Marshall Billingslea, the top U.S. arms control negotiator, is scheduled to meet Monday in Helsinki with his Russian counterpart, Deputy Foreign Minister Sergei Ryabkov, diplomats said. Arrangements for the Helsinki meeting were accelerated after the meeting Friday between Messrs. O’Brien and Patrushev.

Cybersecurity experts said Mr. Trump’s diagnosis created a fertile environment for the rapid spread of disinformation narratives, either by foreign or domestic actors—an issue that was already a top concern ahead of the election.

Zignal Labs, a media analytics firm, said Friday that thousands of tweets on Twitter and hundreds of posts on Reddit were advancing the unsubstantiated claim that Mr. Trump was faking his condition. A less common claim circulating was that Mr. Biden had given the virus to Mr. Trump, Zignal Labs said.

RT, a Russian state media organization, posted a story early Thursday that Mr. Biden was at high risk of contracting the virus because he appeared at Tuesday’s debate alongside Mr. Trump. U.S. intelligence agencies have assessed that Russia is interfering in the 2020 election to denigrate Mr. Biden and support Mr. Trump’s re-election. Mr. Biden and his wife later tested negative for the virus.

Clint Watts, a former Federal Bureau of Investigation official, warned in a blog post in July that one or both candidates contracting Covid-19 shortly before Election Day would represent a “potential disinformation disaster” that could jeopardize faith in the democratic process and make some voters believe a political party was acting to replace the candidate against the will of voters.

“Americans would flock to Covid-19 conspiracies of elite control, further degrading the authority of U.S. institutions,” Mr. Watts, a senior fellow with the Foreign Policy Research Institute, said then.

On Sunday, Mr. Watts said that foreign interference and conspiracy attempts have been light, but that political extremes in the U.S. have been active, especially adherents of the loosely organized, far right-wing conspiracy group QAnon.

“The pro-Trump extreme is blaming his infection on the Democrats giving it to Trump at the debate. The anti-Trump extreme thinks his diagnosis is a means to win sympathy, or avoid the debates,” Mr. Watt said. “There’s a smaller, overlap fringe from both extremes that believes this is a way for Trump to be substituted with Pence. This will surge if the president doesn’t get back on his feet, amongst populists on both sides.”

—Dustin Volz contributed to this article.

Write to Gordon Lubold at Gordon.Lubold@wsj.com