Facebook said Thursday it began implementing a new policy requiring labeling of political ads and verification of identities of those paying for the messages.

The huge social network earlier this year announced the new policy, aimed at improving transparency and countering criticism over its role in propagating Russian-sponsored disinformation during the 2016 US election.

Facebook said that the new policy would be in effect as of Thursday for ads in the United States on Facebook and Instagram. It intends to implement the same policy worldwide in the coming months.

"Starting today, all election-related and issue ads on Facebook and Instagram in the US must be clearly labeled -- including a 'Paid for by' disclosure from the advertiser at the top of the ad," said a blog post by Facebook product management director Rob Leathern.

Facebook will verify the identity of those paying for ads, a move aimed at curbing fake accounts and foreign interference in election campaigns, according to the company.

The policy will apply not only to ads for candidates but to key political issues, which some analysts have said may be difficult to enforce.

Facebook politics and government outreach director Katie Harbath and public policy director Steve Satterfield said the definition of "issue" ads was examined carefully with input from outside parties.

"In the US, there aren't laws or federal agencies that list specific issues that are subject to regulation," they wrote in a separate blog post.

"But to have a policy that our reviewers can enforce, they need a list explaining what's OK and what's not."

Facebook said it developed a list of 20 key issues identified by the Comparative Agendas Project, a non-partisan research center.

These topics include abortion, civil rights, the environment, foreign policy, guns and immigration, according to Facebook.

"We know we'll miss some ads and in other cases we'll identify some we shouldn't," Harbath and Satterfield wrote. "We'll keep working on the process and improve as we go."

The executives said that Facebook debated whether to ban all political ads but decided in the end to keep them with stricter label and verification.

"Political advertising serves an important purpose. It helps candidates share their views with the public more broadly, and it can help encourage people to get involved in the political process," they wrote.

Banning all ads, they maintained "would make it harder for people running for local office -- who can't afford larger media buys -- to get their message out."

Facebook also will be developing a searchable archive of political ads under the new policy.

Outside the United States, Harbath told journalists on a conference call that "we will be working with election regulators and organizers in various countries" as it implements the policy globally.