"We are monitoring the situation in Russia. The events over the weekend are an internal Russian matter, and yet another demonstration of the big strategic mistake that President Putin made with his illegal annexation of Crimea and the war against Ukraine," Stoltenberg told reporters.

He spoke while on a visit to Lithuania -- which will host next month's NATO summit -- a couple of days after the Wagner mercenary group's aborted revolt against the Kremlin.

After Wagner chief Yevgeny Prigozhin called off his troops' advance on Saturday, Moscow said he would leave Russia for Belarus and would not face charges.

"We are also monitoring the situation in Belarus," Stoltenberg said, after Russia sent nuclear ammunition there earlier this month.

"We condemn Russia's announcement about deploying nuclear weapons. This is reckless and irresponsible," Stoltenberg added.

"We don't see any indication that Russia is preparing to use nuclear weapons but NATO remains vigilant."

Lithuanian President Gitanas Nauseda also appeared at the press conference in Vilnius alongside Stoltenberg.

"The events of the past weekend in Russia demonstrated the instability of the Kremlin regime. Similar or even bigger challenges can be expected in the future," he told reporters.

He said the aborted Wagner mutiny could affect the security of the region.

"The establishment of the Wagner group in Belarus may become an additional factor," he added.