BNY Mellon, the sole settlement agent for Treasury securities, disconnected the Chinese bank from the platform after the hack and is waiting for a third party to attest that it is safe to reconnect, the sources said.

The process is likely to go into next week, they said.

The attack, confirmed by ICBC on Thursday, is the latest in a string of demands for ransom that hackers have claimed this year. ICBC Financial Services, the bank's U.S. unit, said it was investigating the attack that disrupted some of its systems, and making progress toward recovering from it.

(Reporting by Lananh Nguyen and Paritosh Bansal; Editing by Megan Davies)

By Lananh Nguyen and Paritosh Bansal