U.S. Federal Reserve Governor Lael Brainard said on Wednesday that Facebook's efforts to launch a Libra cryptocurrency must overcome a "core set of legal and regulatory challenges" before facilitating a single payment.

Brainard added that central banks' efforts to conduct monetary policy could be "complicated" by widespread adoption of an external stablecoin like Libra. But she suggested the Fed is in no rush to issue its own digital currency, saying it raises "profound legal, policy and operational questions."

"It should be no surprise that Facebook’s Libra is attracting a high level of scrutiny from lawmakers and authorities," she said at an event in Washington, according to prepared remarks. "Libra, and indeed any stablecoin project with global scale and scope, must address a core set of legal and regulatory challenges before it can facilitate a first payment."

Policymakers worldwide have continued to express skepticism over Facebook's efforts to build a global digital currency by 2020. That project has been called into doubt of late, as several high-profile companies dropped out of efforts to help establish the currency.

David Marcus, the Facebook (FB) executive leading the currency project, remained confident on Wednesday that the project will attract more backers as regulatory concerns are addressed.

But Brainard said Libra backers have yet to address several major regulatory questions, including how they would ensure the currency is not used for illegal activities across borders. It also is unclear what protections or recourse consumers using the currency would have.

"Consumers need to be cautioned that stablecoins are likely to be starkly different from sovereign-issued currency in legal terms," she said.

Brainard also said stablecoins like Libra could pose financial stability risks, as they could be subject to runs if not managed effectively. 

Facebook's digital currency project has been abandoned by several high-profile partners including PayPal, Visa and Mastercard, still expects to get 100 banks and financial firms on board once it addresses regulatory concerns, Marcus said.

“It will take time for us to address all of the regulatory concerns that were raised and it’s our duty and our responsibility to come with answers to all of these questions,” Marcus told a panel at the International Monetary Fund (IMF) conference in Washington.

“I think once we’ve done this then I think we’ll see more banks and traditional financial services firms join the effort,” Marcus said, adding it was harder for large, regulated financial firms to “take an active part in this fight right now, given the climate and the pressure.”

Facebook’s ambitious efforts to establish a global digital currency have suffered severe setbacks in recent days. Other key members to pull out included Stripe, eBay Inc. (EBAY) and Booking Holdings Inc. (BKNG). PayPal Holdings Inc. (PYPL) said it was leaving earlier this month.

The exodus followed intense skepticism from global policymakers and regulators in the United States and Europe who worry Libra could upend the global financial system, threaten users’ privacy and facilitate money laundering.

Facebook is left without the backing of any major payments firms for the project, due to launch by June 2020.

Speaking at the IMF event, Marcus said around 1,600 entities globally had initially expressed an interest in joining the project and that Libra should have “no problem” reaching its goal of launching with 100 members.

On Monday, the Libra Association’s 21 remaining members forged ahead, holding their inaugural meeting in Geneva and selecting a five-member board.

Marcus said it was now in Libra’s hands to run a process to select more members. He added that Libra also needed time to build up its staff to address the regulatory issues, rather than Facebook “carrying that flag.”