A spokesman also told Die Welt newspaper: "The rumour of a contract extension has been around since the first race in Australia.
"We have not commented so far, and that will continue," he added.
Der Spiegel, meanwhile, said it made a similar enquiry to 28-year-old Rosberg's management, who reportedly 'did not deny' the contract extension news.
"They (Rosberg's management) did not want to comment further," Spiegel added.
Mercedes will hold a press conference in the Monaco paddock on Saturday morning, attended by title sponsor Petronas, Daimler chairman Dieter Zetsche, team boss Toto Wolff and drivers Rosberg and Lewis Hamilton.
Wolff told Der Tagesspiegel newspaper that "conspiracy theories" about Mercedes favouring championship leader Hamilton over Rosberg for the 2014 title are not true.
He insisted their contracts do not give either driver 'number 1' status.
"For us, they are both valuable, with their different personalities," said Wolff. "They both speak to our customers in different ways."
And he said the pair will continue to enjoy a free run to the title, unfettered by team orders.
"Of course, this includes all sorts of mind games and banana skins put in each other's paths, but this is ok so long as they don't throw them at the team," said Wolff.