Ukraine has accused Russia of attacking the besieged city of Bakhmut with phosphorus munitions.
Prigozhin said they would leave on May 10 because of heavy losses and inadequate ammunition supplies.
Russia has launched a series of missiles at Ukrainian cities in the second pre-dawn attack in three days.
The US believes the UN Secretary General is too willing to accommodate Russian interests, according to fresh revelations in classified documents leaked online.
What to make of the dozens of classified American Defence Department documents - maps, charts and photographs - now circulating on the internet?
