UNITED NATIONS (Reuters) - The United Nations said on Wednesday that its offices in Geneva and Vienna were targeted by an "apparently well-resourced" cyber attack in the middle of last year that exposed lists of user accounts, but that the damage had been contained.

Geneva is home to several U.N. arms, including the Human Rights Council, the High Commissioner for Human Rights, the High Commissioner for Refugees, the World Health Organization and the World Trade Organization. Agencies in Vienna include the International Atomic Energy Agency and the Office on Drugs and Crime.

"The attribution of any attack is very uncertain and fuzzy, but this was apparently a well resourced attack," U.N. spokesman Stephane Dujarric told reporters in New York. "The attack resulted in a compromise of core infrastructure components at both (Geneva) and (Vienna), and was determined to be serious."

"The damage related to this specific attack has been contained, and additional mitigation measures implemented. Nevertheless, the threat of future attacks continues, and the United Nations Secretariat detects and responds to multiple attacks of various level of sophistication often," he said.

The U.N. Human Rights Office said in a statement that the servers accessed by the hackers "did not hold any sensitive data or confidential information."

"The hackers did manage to access our Active User Directory, which contains the user IDs for our staff and devices. However, they did not succeed in accessing passwords. Nor did they gain access to other parts of the system," the statement said.

The United Nations is headquartered in New York, where U.N. Secretary-General Antonio Guterres, the U.N. Security Council and U.N. General Assembly are based.

A U.N. spokesman said last week that the world body directed officials in June not to use WhatsApp to communicate because "it's not supported as a secure mechanism." However, some U.N. staff have said WhatsApp is still regularly used by many.

The United Nations was responding to questions about WhatsApp after U.N. experts last week accused Saudi Arabia of using the online communications platform to hack the phone of Amazon Chief Executive and Washington Post owner Jeff Bezos.