Reports that Google has held takeover talks with Twitter that value the microblogging site at as much as $10 billion are just a rumor, Chief Executive Dick Costolo said on Monday.

Asked at the Mobile World Congress fair in Barcelona whether Google could afford a $10 billion acquisition, Costolo replied: I don't know where these things come from. It's just a rumor.

He declined comment on a follow-up question on Facebook, which has also been reported to have held low-level takeover talks with Twitter.

Twitter, which had 175 million users as of September, raised $200 million in a December financing round that valued it at $3.7 billion.

(Reporting by Georgina Prodhan; Editing by Will Waterman)