Nico Rosberg says Mercedes will need to keep an eye on Red Bull this year as they are a "strong team" and could "always make a comeback".

Red Bull slipped to fourth in the Constructors' Championship in 2015 and for the first time since 2008 failed to win a single race.

The team's relationship with Renault also reached a new low and originally it sought to end its agreement with the French manufacturer due to performance and reliability issues. However, after it was blocked from securing a deal with any of the other three engine manufacturers - Ferrari, Mercedes and Honda, it negotiated a new deal that will see it continue to use the Renault power unit, albeit run under the TAG-Heuer RB12 moniker and with the development taken in house with assistance from Ilmor.

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"You always have to reckon with Red Bull," Rosberg said in an interview with Motorsport-Magazin.com. "They are a very strong team. Up to one and a half years ago, they were absolutely dominant, simply unbeatable. They could always make a comeback, so we have to keep an eye on them."

Rosberg also believes - just like Mercedes head of motorsport Toto Wolff - that Ferrari could take another step forward this winter, just like it did twelve months ago.

"We definitely see Ferrari as a threat," Rosberg continued. "They've been making great progress and will be very strong next season.

"They beat us at Sepang and have been competitive ever since."

"But we have consistently been getting better [too]. I'm pleased about that - our team has had a really good development rate," he added.