Microsoft, which accused Motorola a month ago of violating its smartphone patents, filed suit against the US telecom giant again on Tuesday, accusing it of demanding "excessive" royalties.

Microsoft, in the suit filed in Washington state, where the US software giant has its headquarters, charged that Motorola was demanding "excessive and discriminatory" royalties related to patented technology.

A Microsoft spokesman said Motorola was in breach of an agreement to license patents related to wireless and video coding technologies under "reasonable and non-discriminatory terms and conditions."

"Those commitments are designed to benefit all parties that rely upon these standards, and Microsoft has been harmed by Motorola's failure to honor them in recent demand letters seeking royalties from Microsoft," the spokesman said.

In the lawsuit, Microsoft said "Motorola is demanding royalty payments that are wholly disproportionate to the royalty rate that its patents should command under any reasonable calculus.

"Motorola has discriminatorily chosen Microsoft's Xbox product line and other multifunction, many-featured products and software, such as Windows 7 and Windows Phone 7 and products incorporating Microsoft software, for the purpose of extracting unreasonable royalties from Microsoft," the suit said.

Microsoft filed suit against Motorola on October 1, accusing the US handset maker of violating nine Microsoft patents in smartphones powered by Google's Android mobile operating system.

Microsoft supplies its own mobile operating system to handset makers and a new line of Windows Phone 7 smartphones went on sale in the United States this week.