Facebook came under fresh criticism Tuesday for its hands-off approach to political speech, as a group of employees and US lawmakers called on the social network to apply fact-checking for politicians spreading misinformation.

A letter from Facebook employees urged the company to crack down on "civic misinformation," saying the spread of debunked claims is a "threat" to what the company stands for.

"We strongly object to this policy as it stands. It doesn't protect voices, but instead allows politicians to weaponize our platform by targeting people who believe that content posted by political figures is trustworthy," said the letter first obtained by the New York Times, which said some 250 employees had endorsed it. At the same time, US lawmakers critical of Facebook stepped up their calls to revisit its policy, which exempts comments and paid ads on the platform from fact-checking - an issue that has become heated with President Donald Trump's online ads using what some called "provably false" claims.

"Facebook's new ads policy allows politicians to run demonstrably false advertising on its platform. I don't think that's right," said Senator Mark Warner, a Virginia Democrat who added that he sent a letter to Facebook chief executive Mark Zuckerberg calling on him "to reverse this decision."

Other Democrats joined the effort, welcoming the letter from Facebook employees.

"Being a politician shouldn't be a license to lie - especially to spread hatred. If Facebook employees get it so should Zuckerberg," tweeted Senator Richard Blumenthal.

Copyright Agence France-Presse, 2019