Ukrainian defense officials said on Friday they carried out drone attacks on four oil facilities in southern Russia in an effort to disrupt infrastructure supplying enemy forces.

They also said fires at multiple fuel depots in the southwestern region a day earlier were the result of their attacks.

In the Kremlin on Friday, Russian President Vladimir Putin made an argument to maintain nuclear capabilities at a graduation ceremony for military, police and intelligence service academies.

Putin said Russia plans to "further develop the nuclear triad as a guarantee of strategic deterrence and to preserve the balance of power in the world." The "nuclear triad" is a reference to nuclear missiles launched from land, sea and air.

During a recent visit to Vietnam, Putin said he is thinking about possible changes to Russia's nuclear doctrine and strategy.