Crypto exchange FTX saw around $6 billion of withdrawals in the 72 hours before Tuesday morning, according to a message to staff sent by its CEO Sam Bankman-Fried that was seen by Reuters.

In a surprise move, Changpeng Zhao, boss of major rival Binance, said on Tuesday the company signed a nonbinding agreement to buy FTX's non-U.S. unit, FTX.com, to help cover a "liquidity crunch" at FTX.

"On an average day, we have tens of millions of dollars of net in/outflows. Things were mostly average until this weekend, a few days ago," Bankman-Fried wrote in a message to staff sent on Tuesday morning.

"In the last 72 hours, we've had roughly $6b of net withdrawals from FTX," he wrote, adding that withdrawals at FTX's main unit, FTX.com, are "effectively paused," an issue that would be resolved in "the near future."

"Most of the details (of the deal) still aren't hashed out," he wrote, adding that he did not have a "definitive answer" for questions including "what exactly is the transaction" and "what entities would it include."

FTX did not immediately respond to a request for comment.