"Belarus is a sovereign and independent state. And nobody, including Russia, should interfere there," Stoltenberg was quoted as telling Germany's bestselling Bild daily.

Russian President Vladimir Putin earlier Thursday vowed military support for close ally and Belarusian leader Alexander Lukashenko, while urging a peaceful resolution to the unrest and protests that erupted after the disputed August 9 vote.

In an interview with Bild, Stoltenberg said it would be "unacceptable" for the Belarusian regime to use violence to quash peaceful opposition protests.

"President Lukashenko and the regime in Minsk must ensure that the people of Belarus can also decide and determine the future of Belarus," he was quoting as saying, in comments translated into German.

Lukashenko has dismissed calls to resign or host new elections, instead accusing Western countries and Russia of stirring political unrest.

Putin said he would only send in military backing if "the situation starts to get out of control".

But the Russian leader also called on the authorities in Minsk and the opposition to "find a way out" of the crisis peacefully.