Yahoo has notified Facebook that the social networking giant may be infringing on 16 more of its patents.

Yahoo sent a letter to Facebook last month alleging that the patents "may be relevant" to technology Facebook uses in its servers and data centers, Facebook disclosed in a regulatory filing Thursday. Facebook has data centers in Oregon and North Carolina.

Facebook said Yahoo had not threatened a lawsuit but had said "it may do so in the future." The company added that Yahoo's latest move was aiming at open-source and energy-efficient green technologies developed and employed by companies and engineers.

Yahoo sued Facebook in March, alleging that the company was infringing on 10 of its patents. Facebook countersued, asserting that Yahoo was infringing on 10 of its patents.