(Adds Kremlin saying Prigozhin will move to Belarus under deal brokered by Lukashenko, paragraph 9)*Prigozhin says men will return to base*Wagner men start to pull back from Rostov army base**Wagner forces had taken control of RostovROSTOV-ON-DON/VORONEZH, Russia, June 24 (Reuters) - H eavily armed Russian mercenaries who advanced most of the way to Moscow began turning back on Saturday, de-escalating a major challenge to President Vladimir Putin's grip on power, in a move their leader said would avoid bloodshed. The Wagner fighters captured the city of Rostov hundreds of miles to the south before racing in convoy through the country, transporting tanks and armoured trucks and smashing through barricades set up to stop them, video showed. Deputy Defence Minister Hanna Maliar said on Telegram that an offensive was launched near a group of villages ringing Bakhmut, which was taken by Wagner forces in May after months of fighting