Russian army helicopters opened fire on a military convoy of Wagner mercenaries on the M4 highway near the city of Voronezh on Saturday, an eyewitness told Reuters.

Russian military helicopters opened fire Saturday afternoon on a convoy of rebel mercenaries more than halfway to Moscow after capturing the southern city overnight, Reuters reported.

President Vladimir Putin has vowed to crush the armed uprising, which he likens to Russia’s century-old civil war.

Fighters from Yevgeny Prigozhin’s Wagner Group controlled Rostov-on-Don, a city of more than a million people near the border with Ukraine, and were rapidly advancing north through western Russia.

A Reuters journalist saw army helicopters open fire on Wagner’s armed convoy, which was moving past Voronezh in armored personnel carriers.

This comes as Russian security sources told Reuters on Saturday that militants from the Wagner group had taken control of all military facilities in the city of Voronezh, about 500 km south of Moscow. Images shared on social media show the Russians attacking Wagner’s positions.

Russian authorities also announced on Saturday that they were dealing with a major fire that broke out at a fuel depot in the city of Voronezh, the capital of the region of the same name, where Wagner’s paramilitary group is said to be active.

“More than 100 firefighters and 30 pieces of equipment are working on the spot. According to preliminary information, there are no casualties,” said local governor Oleksandr Gusev. He did not specify the cause of the fire, but the media published a video showing a military helicopter in the area before the explosion.

The governor of Russia’s Voronezh region said on Saturday that the army was taking “necessary military measures” in the area as part of an anti-terrorist operation launched after an armed uprising by the Wagner mercenary group.

Footage of Wagner’s alleged attack on the Russian army in Voronezh appeared on social networks. Mercenaries allegedly shot down an Antonov An-26 military transport plane.

In addition, Wagner’s head Yevgeny Prigozhin also said on Saturday that his troops were attacked from helicopters on the track.

The founder of the mercenary group Wagner on Friday accused the Russian command of deadly attacks on the camps of his fighters.

On Saturday, Prigozhin challenged Vladimir Putin and declared that he and his men would not surrender.