In its formal response, Yahoo said its board had concluded Microsoft's unsolicited offer "substantially undervalues" the Sunnyvale-based company.

Yahoo indicated it could be lured to the negotiating table if Microsoft ups the ante, without mentioning the price it has in mind.

"The board of directors is continually evaluating all of its strategic options in the context of the rapidly evolving industry environment and we remain committed to pursuing initiatives that maximize value for all stockholders," Yahoo said in a statement.

Investors appeared confident that Microsoft wants Yahoo badly enough to raise the stakes. Yahoo shares rose 25 cents to $29.45 in Monday's morning trading while Microsoft shares fell 48 cents to $28.08.

Yahoo's stock price had dropped by more than 40 percent in the three months leading to Microsoft's bid, valued at $31 per share when it was announced Feb. 1. The offer was 62 percent above Yahoo's market value at the time.

Many analysts believe Redmond, Wash.-based Microsoft will eventually raise its bid to $35 to $40 per share, sweetening the pot by $5 billion to $12 billion in an effort to negotiate an amicable sale.

Microsoft was prepared to pay at least $40 per share for Yahoo a year ago, according to a person familiar with the talks between the two companies a year ago. Yahoo wasn't interested then because it was confident in its own strategy, said the person, who didn't want to be identified because Microsoft's 2007 offer was never publicly disclosed.

But a higher bid now could hurt Microsoft's own stock price, which has been slipping amid concerns that a Yahoo takeover could be more trouble than its worth. Microsoft's market value has plunged by more than $40 billion, or 14 percent, since the bid was made public.

Microsoft representatives didn't immediately respond to requests for comment Monday morning.