Ballmer, speaking during a seminar at the Cannes Lions International Advertising Festival, said Microsoft had no interest in acquiring Yahoo! but it was still interested in a web search deal.
He said: "We have no interest in acquiring Yahoo! What we have said is that we will continue to remain open to a partnership with Yahoo!"
When asked if a deal could be arranged within the next year, Ballmer replied: "Who knows?"
He gave no further details.
Ballmer has repeatedly said that Microsoft remains interested in forming a web search partnership with Yahoo!
Microsoft proposed to buy Yahoo!'s search assets after the internet firm rejected its $44.6bn (£27.3bn) takeover offer last year.
Reports in January claimed Ballmer met with Yahoo! chairman Roy Bostock, which gave rise to speculation that negotiations for a search deal between the two firms was restarting.
However in recent months there has been no talk of a partnership.
During the Cannes seminar Ballmer also said all traditional media content will be digital within 10 years and publishers are failing to generate serious digital revenues.
As reported by The Guardian, Ballmer said: "All content consumed will be digital, we can [only] debate if that may be in one, two, five or 10 years."
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