Moscow, Russia - The rebel Wagner mercenary force threatened to march on Moscow, Russia, on Saturday before announcing a stunning pull-back, as Kyiv seized on the chaos to launch new assaults against Russian positions in Ukraine.

The Wagner private army captured a key military headquarters in southern Russia and sent a force north to threaten the capital, defying Kremlin chief Vladimir Putin's warning of civil war.

But amid Russia's most serious security crisis in decades, Wagner chief Yevgeny Prigozhin delivered a surprise announcement, saying his troops were turning back to avoid bloodshed in the Russian capital.

"We are turning our columns around and going back to field camps," Prigozhin announced after previously vowing to march on Moscow to topple the military leadership.

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He said understood the importance of the moment and did not want to "spill Russian blood."

Belarusian leader Alexander Lukashenko said he had negotiated a truce with Prigozhin "on stopping the movement of armed individuals from the Wagner group on Russian territory and further steps on de-escalating tensions."

Kyiv reveled in the chaos as Putin's former mercenary ally Prigozhin turned his Wagner force away from the offensive against Ukraine and made threats to topple the chiefs of Russia's military.

"The man from the Kremlin is obviously very scared and is probably hiding somewhere," Ukraine's President Volodymyr Zelensky said in his daily address, adding that Putin has "created this threat himself."

Separately, Ukraine's deputy defense minister Ganna Malyar announced that Ukrainian forces had gained more ground in the eastern region of the Donbas, launching new counteroffensives in several areas.

Putin's spokesman insisted the Russian leader was still at work in the Kremlin and had not fled Moscow.