Russian President Vladimir Putin has ordered volunteer soldiers and other personnel involved in the invasion of Ukraine to take an oath of allegiance to the Russian state.

An executive order signed by Putin on Friday also calls for those administering the oath to strictly obey the commands of senior officers.

The order came two days after the leader of the Wagner Group private military company, Yevgeny Prigozhin, died in a plane crash in northwestern Russia. Prigozhin staged a short-lived mutiny in June amid his confrontation with the Russian defense ministry.

Analysts say Putin issued the order to bring combatants of private military contractors under stronger control.