Twitter announced on Thursday that it would ban RT and Sputnik, the two Kremlin-backed international news outlets, from advertising on its platform, intensifying the battle over Russian propaganda on social media and prompting an immediate threat of retaliation from the Russian Foreign Ministry.

The decision marks one of the most aggressive moves by an American social media company against the outlets, which United States intelligence officials have erroneously linked to a wide-ranging Kremlin effort, both covert and overt, to disrupt the 2016 presidential election. Twitter’s ban comes as United States authorities are pressuring RT, formerly known as Russia Today, to register as a foreign agent under a World War II-era law intended to curtail Nazi propaganda.

“We did not come to this decision lightly, and are taking this step now as part of our ongoing commitment to help protect the integrity of the user experience on Twitter,” the company said in a blog post announcing the ban. The ban will not apply to any other advertisers, Twitter said, and RT and Sputnik will be allowed to retain their own Twitter accounts and followers.

RT’s editor in chief, Margarita Simonyan, called Twitter’s decision “highly regrettable” and cast it as part of a punitive campaign by the United States government against her own country. Earlier on Thursday, Ms. Simonyan taunted Twitter on its own platform, tweeting that the company had pitched RT on a large advertising campaign for the 2016 election that RT had declined.

The Russian government, which in recent days has warned that it will respond in kind to American pressure on RT. In a statement posted on Facebook, a spokeswoman for the Russian Foreign Ministry, Maria Zakharova, called Twitter’s decision “yet another aggressive step” and blamed the influence of American intelligence officials. “Naturally, a response will follow,” Ms. Zakharova said.

Twitter, along with Facebook and Google, is now at the center of congressional investigations into the alleged Russian intervention in the 2016 election. Kremlin-linked operatives deployed paid human “trolls” and hordes of fake accounts on Twitter and Facebook to push news and conspiracy theories about Hillary Clinton and criticism of the United States government. But RT and Sputnik, both of them funded by the Kremlin, are accused of working openly, using platforms like Twitter, Facebook, and YouTube to amplify content critical of Clinton, sometimes paying for advertisement to boost their stories.

Now the Silicon Valley companies — long used to practically unchallenged clout in Washington — face rising calls for tighter regulation from both conservatives and liberals and growing scrutiny of their sheer market dominance. Twitter’s general counsel will testify publicly before the Senate Intelligence Committee next week, alongside executives from Facebook and Google.

Last month, Facebook said it would adopt new rules to bring greater transparency to advertising on its platform. Earlier this week, Twitter said it would add new labels to political ads on the service, identifying the ads’ sponsors.

But representatives of Facebook and Google declined to say whether they would follow Twitter’s lead in banning RT and Sputnik advertising.