The World Bank has severed electronic links with the International Monetary Fund after warnings that IMF computer systems had been infiltrated by an unknown foreign power.
Officials at the fund gave few details but said the attack earlier this year had been "a very major breach" of its systems, the New York Times reports.
The newspaper said IMF staff had been told of the intrusion on Wednesday in an e-mail which warned that "suspicious file transfers" had been detected. The intruders are understood to have gained access through a phishing e-mail which planted a malware spy on the Fund's desktop systems.
The IMF has confirmed the breach and is working with the FBI to investigate the incident.
The World Bank said it severed links with the IMF "out of an abundance of caution".
World Bank spokesman Rich Mills says: "The World Bank Group, like any other large organisation, is increasingly aware of potential threats to the security of our information system and we are constantly working to improve our defences."
The cyber-intrusion is the latest in a spate of sophisticated hacking offences that have breached the defences of major banks, corporations and government agencies worldwide.