Microsoft has released six patches to fix software flaws that carry its highest threat rating, including three for defects that attackers were already trying to exploit.

The company said all six of the critical flaws could allow an attacker to obtain some access to other people's computers.

The Redmond-software maker also released four other patches to fix vulnerabilities that the company deemed less severe.

Customers can download all the patches for free on Microsoft's security website and also can sign up to have them automatically delivered to their computers.

The automatic update system went down for several hours last week, but the problem was later resolved.

Microsoft said last month that it knew attackers were already trying to take advantage of defects in its Windows operating system, Microsoft Word software and PowerPoint presentation program.

Christopher Budd, a program manager with the Microsoft Security Resource Centre, said that the company had seen limited attacks exploiting the flaws, but were nevertheless recommending that users apply those and other patches immediately.

Microsoft software is a constant target of Internet attackers, in part because the company's products are so widely used. Microsoft has yet to release a patch for one other publicly known flaw - one affecting the Internet Explorer browser that is part of its Windows operating system. Mr Budd said the company was seeing very few attacks as a result of the flaw.