Facebook on Monday estimated that as many as 10 million people saw the political advertisements that were purchased by a shadowy Russian internet agency and ran on its platform.

The company made the announcement after turning over 3,000 ads to congressional investigators examining Russian interference in the US election.

Elliot Schrage, Facebook’s vice-president of policy and communications, said the advertisements appeared to focus on “divisive social and political messages across the ideological spectrum, touching on topics from LGBT matters to race issues to immigration to gun rights”.

Less than half of the ads were seen prior to the US election on 8 November, Schrage said in the post, while 56% were viewed after. And roughly a quarter of the ads were not seen by anyone. On 99% of the ads, less than $1,000 was spent, he said.

He also said that some of the ads were paid for in Russian currency, but said that distinction alone was not a “good way” of identifying suspicious activity. Schrage defended the company’s use of targeted advertising but said “certain types of targeting will now require additional human review and approval”.

Facebook, along with Twitter and Google, is under mounting pressure from lawmakers and the public to disclose more details about Russia’s use of the platforms to spread disinformation and propaganda as part of an influence campaign ahead of the elections – a notion Facebook’s CEO, Mark Zuckerberg, initially dismissed as “crazy”. Last week, Zuckerberg apologized: “Calling that crazy was dismissive and I regret it. This is too important an issue to be dismissive.”

On 6 September, Facebook revealed a Russian-based influence operation had purchased $100,000 worth of ads to promote divisive political and social messages during the presidential campaign. The ads, which ran between June 2015 and May 2017, spread controversial views on topics such as immigration and race and promoted 470 “inauthentic” pages and accounts, according to Facebook’s own description. The disclosure has added fuel to the findings of US intelligence officials that Russia was involved in influencing Trump’s race against Hillary Clinton.

“The 2016 US election was the first where evidence has been widely reported that foreign actors sought to exploit the internet to influence voter behavior,” Schrage wrote.

“We understand more about how our service was abused and we will continue to investigate to learn all we can. We know that our experience is only a small piece of a much larger puzzle. Congress and the Special Counsel are best placed to put these pieces together because they have much broader investigative power to obtain information from other sources.”

Facebook shared the 3,000 ads with the House and Senate intelligence committees and the Senate judiciary committee. The company had already turned over the ads to the special counsel Robert Mueller.

In an earlier post on Monday, Joel Kaplan, Facebook’s vice-president of global public policy, said the company was planning to hire more than 1,000 people to help review advertisements globally. Kaplan said the company would require more documentation from advertisers who want to run advertisements related to the US election.