The owner of the Wagner private military contractor who called for an armed rebellion aimed at ousting Russia’s defense minister, confirmed Saturday morning that he and his troops have reached a key Russian city after crossing the border from Ukraine.

Yeveny Prigozhin posted a video of himself in Rostov-on-Don at the Russian military headquarters that oversees the fighting in Ukraine. He claimed that his forces had military facilities in the city under their control, including the air field. Other videos posted on social media showed military vehicles, including tanks, on the streets outside.

Russia's security services had responded to Prigozhin's declaration of an armed rebellion by calling for his arrest. In a sign of how seriously the Kremlin took the threat, security was heightened in Moscow and in Rostov-on-Don. It was not immediately clear how he was able to enter the southern Russian city or how many troops he had with him.

While the outcome of the confrontation was still unclear, it appeared likely to further hinder Moscow’s war effort as Kyiv’s forces were probing Russian defenses in the initial stages of a counteroffensive. The dispute, especially if Prigozhin were to prevail, also could have repercussions for President Vladimir Putin and his ability to maintain a united front.

The Wagner forces have played a crucial role in Russia’s war in Ukraine, succeeding in taking the city where the bloodiest and longest battles have taken place, Bakhmut. But Prigozhin has increasingly criticized Russia’s military brass, accusing it of incompetence and of starving his troops of weapons and ammunition.