The credit card giants are reportedly wavering in their support for the internet payments network amid a backlash from US and European officials.
Facebook's poor privacy record has not sat well with regulators, who are set to impose tough rules on Libra.
Keen to keep these regulators on side, Visa and Mastercard executives are pondering pulling out of the project, sources told the Wall Street Journal.
But Libra co-creator David Marcus hit back at these reports and said he was not aware of any company set to withdraw.
He tweeted: 'For Libra to succeed it needs committed members, and while I have no knowledge of specific organizations plans to not step up, commitment to the mission is more important than anything else.'
And he added: 'The tone of some of this reporting suggests angst, etc... I can tell you that we're very calmly, and confidently working through the legitimate concerns that Libra has raised by bringing conversations about the value of digital currencies to the forefront.'