Microsoft plans to patch a newly discovered hole in Microsoft Word in its next monthly patch update, and may release a rare "out of cycle" patch to address the hole, a company spokesman said.

Microsoft's Security Research Centre is analysing the previously unknown vulnerability, which affects Microsoft Word XP and Word 2003 and is already being linked to targeted Internet attacks on government agencies in the U.S. and European Union, as well as U.S. government defence contractors, according to security experts.

The attacks, which use Word attachments to very targeted "phishing" e-mail messages, are being likened to a widespread attack on the U.S. government last year that was code named "Titan Rain."

Anti-virus firms warned of the unknown Word hole on Friday, after attacks were first detected. When a malicious Word document is opened, an exploit file installs a Trojan horse program on the affected Windows system with a rootkit component that makes the malicious program almost undetectable.

According to research by F-Secure, the Trojan communicates with a host registered at 3322.org, a free hosting service based in China.

Microsoft is completing development of a security update for Microsoft Word that addresses the vulnerability. The fix is being tested and could be released on June 13, with the company's scheduled monthly patch release, or sooner, according to an e-mail sent from a Microsoft spokesman.

The attacks, or others like them, have been going on for months, and combine malicious Word attachments with e-mail messages that are targeted at individual users within government agencies and corporations, said Johannes Ullrich, CTO at the SANS Internet Storm Center.

Sometimes referred to as "spear phishing" attacks, the e-mail-borne threats are sent from outside an organisation but made to appear as if they come from within the company.

Because the e-mail attacks are sent to one or two specific users, they are very hard to spot using automated tools, and often use the names of actual employees within an organisation and other tricks to deceive recipients into opening the malicious attachments, Ullrich said.

Previous attacks have also buried attacks in Word documents, which are commonly sent between employees and businesses, and rarely blocked at the network gateway. Often those attacks used known vulnerabilities in Word, but were repackaged in ways that fooled anti-virus scanners, Ullrich said.

The latest attacks are similar to an earlier, coordinated attack on U.S. government computer systems that has been dubbed "Titan Rain," which were also believed to have originated in China, Ullrich said.