Facebook says it has banning foreign advertisements related to Ireland's abortion referendum amid concerns that North American groups are trying to influence the campaign.

Irish voters will decide 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. U.S.-based anti-abortion groups are among those who have bought online ads in Ireland during the campaign.

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

Facebook has tried to improve its transparency 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.