Twitter (TWTR) said Thursday it is banning advertisements on its platform from two Russian media firms, Russia Today and Sputnik, citing an internal investigation and findings by intelligence authorities that the companies sought to interfere in the 2016 US presidential election.

The social media company also said it would donate $1.9 million, the amount Twitter estimated it has earned from RT since 2011, to help fund research into potential abuses of the microblogging service in elections and other civic activities.

"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 statement.

The ad restriction applies only to RT, an English-language TV broadcaster that receives financial support from the Russian government, and Sputnik, a news agency and online news outlet.

Get Breaking News Delivered to Your Inbox

On its website, Sputnik's editor-in-chief called Twitter's decision "regrettable."

In a story published on its website, RT denied trying to interfere in the US election, saying that it "has never been involved in any illegal activity online."

"RT has never spread any sort of deliberate misinformation," the company said. "If we had, RT would have been banned by any social media platform in question, as all of them have strict rules regarding policy violations."

RT said Twitter approached the media company in 2016 to discuss a potential advertising deal to promote RT's U.S. election coverage on the blogging service. RT said it declined the proposal.

In another article on Thursday, RT cited a spokeswoman for the Russian foreign ministry who called Twitter's move a "blatant violation" of international norms and threatened retaliation.

"We consider this a yet another aggressive step aiming to block the activity of Russian channel RT," Foreign Ministry spokeswoman Maria Zakharova said in a statement to the media company.

Twitter, Facebook (FB) and Google parent Alphabet (GOOGL) face scrutiny as part of an ongoing probe into Russian meddling in last year's U.S. presidential election. Representatives from the companies are scheduled to appear before a House intelligence panel next week to answer questions about their roles in such interference.

Twitter said on Tuesday that it will provide more information about political ads that run on the platform, including who is funding them and whom they are aimed at.

Facebook has also agreed to disclose ads to Congress that were purchased by Russians on the social media platform. The social media giant said in September that roughly 3,000 ads connected with 470 fake accounts ran on Facebook between June of 2015 and May of this year. Some of those accounts, which targeted U.S. voters before and after the 2016 election, were linked to a Russian group called the Internet Research Agency that has been identified as a "troll farm."