The firm has handed over around 3,000 ads to investigators working for the US Congress.

It said the subject of the ads included immigration issues, gun rights and LGBT topics.

They focused on "divisive social and political messages", it said.

In a blog post, Elliot Schrage, vice-president of policy and communications, wrote that he believed around 10 million people would have seen some of the adverts, but that 25% of them would not have been seen by anybody.

Virtually all (99%) were bought for less than $1,000 (£760) each, with half of those costing less than $3.

However around $6,700 was spent on the adverts which appeared on Instagram.

Facebook has announced changes to the way adverts are signed off, including "additional human review and approval" for some targeted ads.

It added that many of the adverts it shared with Congress did not violate its policies.

The US is carrying out a wide-ranging investigation into whether Russia interfered with the 2016 presidential election.

Google did not directly confirm a report by the Washington Post which claimed that the tech giant had also uncovered advertising bought by suspected Russian agents across its platforms, including YouTube and Gmail, in an attempt to meddle with the election.

"We have a set of strict ads policies including limits on political ad targeting and prohibitions on targeting based on race and religion," said a spokesperson.

"We are taking a deeper look to investigate attempts to abuse our systems, working with researchers and other companies, and will provide assistance to ongoing inquiries."