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Microsoft has made an unsolicited bid for Yahoo, but after the internet company released better than expected first-quarter results earlier this week, there was speculation that Microsoft may raise its bid.

"We know what Yahoo is worth to us," Ballmer said onstage after an address at an information technology event. "We offered a lot of money: $44 billion. If its board thinks that's fair, great. If not, we'll move forward." He didn't explain what he meant by moving forward.

Ballmer later told a news conference, "Time is money. We made clear in the last letter we sent [to Yahoo's board]. We'll see what happens."

Yahoo faces a Saturday deadline to respond to Microsoft, after which Microsoft has said it will launch a proxy battle to unseat Yahoo's board.

Microsoft wants to buy Yahoo to challenge Google in online advertising. Asked whether Microsoft could make a bid for Google, Ballmer said that he did not think the internet company would be interested and a bid would raise regulatory issues.