Israel's Calcalist financial newspaper reported that Viber was in talks to be acquired by a leading instant messaging company from Asia for $300 million-$400 million.
Viber is run from Cyprus by Israeli entrepreneur Talmon Marco and has development centres in Belarus and Israel.
"I have no idea what this is about," Marco told Reuters, adding that the company was not in talks to sell.
Asia has several mobile chat applications such as Chinese WeChat, Japanese Line, and KakaoTalk from South Korea.
Viber has about 200 million registered users in 193 countries. It launched an instant messaging app for personal computers that allows users to make outgoing mobile calls to other Viber users and to non-registered mobiles, making it a rival to Skype.
The company is funded from the pockets of its founders and several private investors from the United States, Calcalist said.
Viber was last in news with its app update for the Windows Phone platform in late January, which brought new features and performance improvements.
Viber 4.0 for Windows Phone added 'Seen' status that lets users know their messages have been viewed, and a 'Last online' timestamp that lets users know when contacts were last seen on Viber. Photo handling has been improved to include the ability to add description to photos and improved quality of photo thumbnails in conversation.
The app brings an enhanced notification system with users getting the ability to hide notification content, if needed. Also included are new Viber notification sounds that can be used for incoming push notifications. Read our previous coverage for more details on the Viber 4.0 update for Windows Phone.
Written with inputs from Reuters
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