Facebook is banning foreign advertisements relating to Ireland's abortion referendum amid fears US groups are trying to influence the result.

Irish voters will decide on May 25 whether to repeal a constitutional ban on abortion - a referendum that has drawn international attention.

Ireland bars political donations from abroad, but the law does not apply to social media advertising.

Facebook is banning foreign advertisements relating to Ireland's abortion referendum amid fears US groups are trying to influence the result. Pictured: Company CEO Mark Zuckerberg

US-based anti-abortion groups are among those who have bought online adverts in Ireland during the campaign.

Facebook says starting on Tuesday it will 'begin rejecting ads related to the referendum if they are being run by advertisers based outside of Ireland'.

It is the latest move by Facebook to boost the transparency of its political advertising with the social media giant under scrutiny for its role in Britain's Brexit referendum and the 2016 US presidential election.

It also comes after revelations that political consultancy Cambridge Analytica harvested users' data to micro-target political ads to select groups during the 2016 U.S. presidential race.

Facebook says starting Tuesday it will 'begin rejecting ads related to the referendum if they are being run by advertisers based outside of Ireland'.

'As part of our efforts to help protect the integrity of elections and referendums from undue influence, we will begin rejecting ads related to the referendum if they are being run by advertisers based outside of Ireland,' said Facebook in a statement on its website.

The company said it would rely on reports from established campaign groups on both sides of the campaign to identify foreign-based ads, as its automated election integrity tools are still in development.

On April 25, the company launched a trial of a 'view ads' tool, which allows users to view all of the ads any advertiser is running on Facebook in Ireland at the same time.