Carl Icahn and Yahoo's board traded barbs once again on Friday as the billionaire investor increased his pressure on Yahoo to make a deal with Microsoft, demanding that the company put itself up for sale to Microsoft for a little more than $34 a share.

In a letter Friday to Yahoo Chairman Roy Bostock, Icahnn told the board to stop avoiding the issue and publicly offer to sell the company to Microsoft for $34.375 a share, according to a report in The Wall Street Journal, which posted the letter on its Web site.

[ For the complete saga of Microsoft's attempted takeover of Yahoo, check out InfoWorld's special report ]

The total would be about $49.5 billion, about $5 billion more than Microsoft's original Feb. 1 bid for the company and $2 billion more than the last offer Microsoft made before taking the deal off the table early last month.

"In my opinion, Microsoft does not believe you will ever sell the entire company on a friendly basis," Icahn wrote in the letter, according to the WSJ. "So why don't you stop dancing around the subject and publicly offer to sell the company to Microsoft for $34.375 per share and promise to cooperate completely?"

Yahoo's board responded immediately and harshly, accusing Icahn of having "no credible plan to operate Yahoo" and calling his demand that the company put itself up for sale publicly "ill-advised."

"We believe that Mr. Icahn's suggestion that we cancel our retention plan would have a destabilizing impact on Yahoo and would clearly not be in the best interest of our shareholders," Yahoo said in a statement.

Yahoo added that the company is still "open to any transaction including a sale to Microsoft if it is in the best interests of shareholders."

However, it's not clear if Microsoft is still interested in purchasing the company. Microsoft's last public statement on a possible deal was on May 18, when the company said it would consider a deal to purchase some but not all of Yahoo, but was not making a new bid to purchase the entire company.

Friday's exchange continues a week of letters between Icahn and Bostock over the proxy battle Icahn officially mounted against Yahoo on May 15, and pressure the investor continues to put on Yahoo's board to make a deal with Microsoft.

Icahn aims to remove Yahoo's board members, including Bostock and CEO Jerry Yang, at an August shareholder meeting and replace them with his own slate of directors. In a letter to Yahoo Tuesday, Icahn said he also planned to oust Yang as CEO if he is successful in removing the board.

In Friday's response to Icahn's latest missive, Yahoo also called Icahn's interpretation of a Yahoo employee retention package "inaccurate," reiterating comments made by Bostock in a letter to Icahn late Wednesday that rejected the investor's charges the package was designed to discourage a Microsoft takeover.

Yahoo adopted a new employee severance package after Microsoft announced its $44.6 billion hostile takeover bid on Feb. 1. The package would have cost Microsoft up to an additional $2 billion, offering employees terminated or who resigned for "good reason" four months to two years severance pay depending on their position.

Icahn viewed the retention plan as a "poison pill" -- usually a corporate maneuver designed to stave off a hostile takeover -- and told Bostock as much in a letter Wednesday, to which Bostock responded later that day.

Microsoft walked away from its attempt to purchase Yahoo on May 3 after three months of failed negotiations between the two companies. Since then Icahn and other shareholders irked by the company's inability to make a deal have sought various means of redress, accusing Yang, Bostock and the board of not protecting their interests.