Microsoft has stepped up as a potential bidder for Yahoo!, making a deal to review the struggling Internet pioneer's financial books, The New York Times reported on Wednesday.

In signing a non-disclosure agreement, according to the Times, Microsoft has joined a small pool of suitors that includes private equity firms that appear to be preparing serious bids for Yahoo!.

Microsoft has reportedly been collaborating with private investors to assemble a multi-billion-dollar offer to purchase the Internet firm that spurned the US software giant's takeover effort in 2008.

Microsoft has since arranged for its Bing search engine to power queries at Yahoo! websites and to share in revenue from online advertising sold by the Sunnyvale, California-based firm.

Microsoft has reason to protect its relationship with Yahoo! by not letting it fall into the hands of a potential competitor, according to analysts.

At least nine private equity firms are reported to be eyeing Yahoo! and its global audience of 700 million monthly visitors to the company's various websites, including Yahoo! News, Yahoo! Finance and Yahoo! Sports.

If Microsoft were to succeed in buying Yahoo! it is believed it would spin off the company's Asia assets, which include online commerce titan Alibaba.

Alibaba is 43 percent owned by Yahoo! and is considered one of its best assets, but the Chinese firm reportedly feels it has outgrown its US partner.

The relationship between the two companies was strained earlier this year in a dispute over Alibaba's online payments platform Alipay.

Alibaba Group Chairman Jack Ma called on Yahoo! last month for an answer to his long-standing offer to buy all or part of the US Internet giant, saying delays were hurting both firms.

Once seen as the Internet's leading light, Yahoo has struggled to build a strongly profitable, growing business out of its huge Web presence and global audience.

Yahoo! continues to search for a new chief executive, after it fired Carol Bartz in September, under three years after she was brought in to help turn around the struggling Internet company.

Microsoft was publicly humiliated in 2008 when Yahoo! co-founder Jerry Yang rejected a generous bid for the company at $33 a share, a valuation of more than $47 billion.

At the time a frustrated Microsoft chief Steve Ballmer emphatically said he was through with Yahoo! acquisition talks.

Meanwhile many Yahoo! shareholders blasted the company for spurning Microsoft, contributing to Yang's decision to step aside as chief executive.