This could get ugly. The deadline that Microsoft set three weeks ago for Yahoo to accept its buyout offer passed this weekend without either side making a move, Fortune reports.

"The three-month-old battle between Microsoft and Yahoo could come to a head this week. Yahoo has repeatedly rebuffed Microsoft and demanded a higher price, a step that Ballmer has said he won't take."

The report said that while some analysts expect Microsoft to launch a hostile bid early this week, it's possible that CEO Steve Ballmer will follow through on his threat and walk away from the deal. "Our board and management team continue to be open to any and all alternatives, including a sale to Microsoft," Yahoo CEO Jerry Yang said last week, when Yahoo reported first-quarter earnings that beat estimates.