Microsoft fired a broadside at Google on Monday in a battle over a US government contract, accusing its rival of issuing misleading claims.

Google rejected the charges by the Seattle-based software giant in the legal tussle over a nearly $60 million contract with the US Department of Interior.

Microsoft said documents unsealed in the court case showed that "Google Apps for Government," Google's Internet-based suite of office tools, had not been certified under the Federal Information Security Management Act (FISMA).

"Given the number of times that Google has touted this claim, this was no small development," Microsoft deputy general counsel David Howard said in a blog post. "It's time for Google to stop telling governments something that is not true."

FISMA lays out the security standards for information management systems such as email.

Google and Onix Networking Corp., a reseller of Google products, filed suit against the US government last year claiming the terms for the Department of Interior contract favored Microsoft.

Google argued that the terms of the bid for an email, calendar and document collaboration system for some 88,000 Interior Department employees implicitly rule out a Google product and favored one from Microsoft.

Google was promoting Google Apps for Government for the contract over Microsoft's solution.

Google said Monday in response to Microsoft's accusations that a version of Google Apps has already received FISMA clearance.

"We did not mislead the court or our customers," Google said in a statement. "Google Apps received a FISMA security authorization from the General Services Administration in July 2010.

"Google Apps for Government is the same system with enhanced security controls that go beyond FISMA requirements," Google said.

Relations between the technology giants have become increasingly acrimonious of late and Microsoft last month joined an anti-trust complaint in Europe against Google over Internet search.

In February, Google accused Microsoft's search engine Bing of copying its results.