Yahoo shares were off 13% in midafternoon trading Friday after Microsoft CEO Steve Ballmer said his company is no longer interested in acquiring the Internet portal.

"We made an offer, we made another offer, and it was clear that Yahoo didn't want to sell the business to us and we moved on," said Ballmer, according to the Associated Press. "We are not interested in going back and relooking at an acquisition," Ballmer said.

Ballmer spoke Friday at a business lunch in Sydney, Australia.

The news disappointed investors, many of whom thought Microsoft might renew its campaign to acquire Yahoo after Yahoo and Google earlier this week said they would dissolve their search advertising partnership.

Yahoo shares were down 13% to $12.15 shortly after 2 pm. At one point earlier this year, Microsoft offered to acquire Yahoo for $33 a share -- a bid that was rejected by Yahoo's board. The rejection sparked an effort, led by corporate raider Carl Icahn, to overthrow the board and CEO Jerry Yang. The plot didn't succeed.

Speaking in Sydney, Ballmer said he hasn't ruled out working with Yahoo altogether. "I'm sure there are still some opportunities for some kind of partnership in search, but I think an acquisition is a thing of the past," Ballmer said, according to the AP.

Meanwhile, the U.S. Department of Justice welcomed Google's decision to abandon its search partnership with Yahoo. The agency said the move ensures that the market for Internet search advertising would remain competitive.

"The arrangement likely would have denied consumers the benefits of competition -- lower prices, better service and greater innovation," said Thomas Barnett, assistant attorney general for antitrust, in a statement.

In afternoon trading Friday, Microsoft shares were up 2.1% to $21.32, while Google shares were up a half-percent to $332.83.