American depositary shares of French online advertising company Criteo SA rallied in unusually heavy Nasdaq trading Friday, after French media reported that advertising group Publicis Groupe SA was in talks to buy the Paris-based company.

The advance held even after a source close to Publicis told Reuters that the reports were "unfounded".

Financial daily Les Echos reported on Friday that negotiations, which first began before a planned $35 billion merger of Publicis and Omnicom Group Inc was unveiled in July 2013, restarted three months ago after the Omnicom deal fell through.

Boursier.com, a financial website for traders, had earlier cited an unnamed market source as saying Publicis was considering making a bid for Criteo.

Criteo has a market capitalization of roughly $1.8 billion. Even with a 20 percent premium, the price tag would barely top $2 billion and Publicis could easily afford it, Les Echos said.

Criteo and Publicis declined to comment.

Exane BNP Paribas analyst Charles Bedouelle said that few other "meaningful" acquisitions were possible in this market, where Criteo is one of the "very best and largest" players.

Criteo, which went public last October, raised its full-year revenue forecast earlier this month.

Publicis warned last month it would be "very difficult" to meet its sales growth target this year after a second-quarter slowdown, caused in part by the failure of its planned merger with Omnicom in May.

The world's third-largest advertising group and its deal with No. 2 Omnicom was supposed to create the world's largest agency.

The companies called off the merger after a battle for control and divergent corporate cultures.

According to unofficial figures, Criteo shares closed up 22 percent at $37.46, their biggest-ever gain.

Trading of more than 7 million shares was about 15 times the daily average, making the stock one of the most traded on the Nasdaq.