Microsoft chief executive Steve Ballmer set a three-week deadline as he threatened Saturday to go directly to Yahoo's shareholders to advance his company's efforts to take over the Internet giant.

"We believe now is the time for our respective companies to authorize teams to sit down and negotiate a definitive agreement," Ballmer said in a letter to Yahoo's board of directors and posted on Microsoft's website.

Yahoo has spurned Microsoft's two-month-old offer.

"If we have not concluded an agreement within the next three weeks, we will be compelled to take our case directly to your shareholders, including the initiation of a proxy contest to elect an alternative slate of directors for the Yahoo board."

Ballmer complained that the two sides have made entered "no meaningful negotiation to conclude an agreement" on Microsoft's unsolicited February 1 offer of 31 dollars a share for Yahoo, a 62 percent premium on the closing price of the time.

"Our goal in making such a generous offer was to create the basis for a speedy and ultimately friendly transaction. Despite this, the pace of the last two months has been anything but speedy," he wrote.

He added that in the past two months the US economy and stock markets have weakened considerably.

In addition, he said, "public indicators suggest that Yahoo's search and page view shares have declined."

He also accused Yahoo's board of adopting new plans that will raise the cost of Microsoft's takeover, if successful.

"By any fair measure, the large premium we offered in January is even more significant today. We believe that the majority of your shareholders share this assessment," he said.

"The substantial premium reflected in our initial proposal anticipated a friendly transaction with you. If we are forced to take an offer directly to your shareholders, that action will have an undesirable impact on the value of your company from our perspective which will be reflected in the terms of our proposal," the letter said.

"We think it is critically important not to let this window of opportunity pass."