Facebook needs to stop paying lip service to the fight against disinformation and take more concrete actions, the Mozilla Foundation said in a letter posted Monday.

The letter was co-signed by the Open Data Institute and 30 other groups representing human rights advocates, journalists and technologists. It charges that CEO Mark Zuckerberg's company has been dragging its feet and blocking transparency tools ahead of the EU's parliamentary elections in May.

"Promises and press statements aren’t enough; instead, we need to see real action over the coming months, and we will be exploring ways to hold Facebook accountable if that action isn’t sufficient," the letter says.

Facebook has come under fire since the 2016 U.S. presidential election for not moving fast enough to stop the flow of fake news and misinformation, for the tactics it employs against enemies and for its leadership structure.

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The letter calls out what it claims are contradictory statements and actions emanating from Facebook, including its restrictions on who can monitor political advertisements placed on the network and the tech giant's reported efforts to block access to transparency tools.

"In the company’s recent Wall Street Journal op-ed, Mark Zuckerberg wrote that the most important principles around data are transparency, choice and control," the letter states. "By restricting access to advertising transparency tools available to Facebook users, you are undermining transparency, eliminating the choice of your users to install tools that help them analyze political ads, and wielding control over good faith researchers who try to review data on the platform."

The letter demands the following from Facebook: Roll out a functional open Ad Archive API to permit advanced research and the development of tools to analyze Facebook ads shown to European Union users. Ensure all political advertisements are easy to distinguish from other content and accompanied by extensive targeting criteria. Stop harassing researchers who are creating tools to provide more transparency into the online giant's advertising platform.

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"We believe that Facebook and other platforms can be positive forces that enable democracy, but this vision can only be realized through true transparency and trust," the letter concludes. "Transparency cannot just be on the terms with which the world’s largest, most powerful tech companies are most comfortable."

When contacted by Fox News, Facebook provided a statement from Rob Leathern, the company's director of product management, and pointed toward recent blog posts on the topic:

“We’re committed to a new level of transparency to ads on Facebook and encourage others to do the same. That’s why we’re planning to open the Ad API in late March -- at the same time as our already-announced program to label political ads ahead of the European Parliament election -- so people on and off Facebook can analyze political ads and see them archived in a library for up to seven years. This work is important and part of our broader efforts to protect elections this year. We’re also bringing transparency tools for political ads to India, Israel, Ukraine, and globally in June.”