UNITED NATIONS — Computer hackers scrawled anti-American and anti-Israeli messages on several U.N. Web sites over the weekend, exposing serious security holes in the international body’s computer system.

The hackers were able to infiltrate the system, U.N. spokeswoman Michelle Montas said yesterday, by using what she referred to as “pseudonyms.” Their messages were posted on the sites of several U.N.-related agencies, including the page that posts Secretary-General Ban’s speeches. The messages do not represent the secretary-general’s views, Ms. Montas said. Containing no punctuation marks, the messages left on the sites were variations on the following: “Hacked By kerem125 M0sted and Gsy That is CyberProtest Hey Israel and Usa dont kill children and other people Peace for ever No war.”

A similar message was also typed in uppercase letters. On Sunday morning, when the messages first began to appear, a U.N. spokesman told a Web site that follows the organization’s affairs, Inner City Press, “We are reviewing to ensure that the security of our Internet services will be improved.” The Inner City Press site, however, cited “skeptics” who said the “outdated” U.N. Web system remains vulnerable to hackers.

As late as yesterday afternoon, at least one site, belonging to the U. N. Environmental Program, was unable to rid itself of a message attacking “Ysrail and Usa.”

Ms. Montas said that, beyond the aliases that were posted on the messages, the United Nations has no idea about the identity of the hackers or whether any of them was a U.N. staffer.

The most advanced computer security companies in the world are based in America and Israel, according to experts.