The developments have prompted concern in the U.S., which has dispatched a slew of senior officials to counter Putin’s advances in an OPEC member that European governments are courting as a potential alternative to Russian energy.

The Kremlin promised unspecified retaliation for what it termed a ‘terrorist’ act, and pro-Kremlin figures called for the assassinations of senior Ukraine leaders.

The visits to annexed territory were a show of defiance by the Russian leader two days after a court issued a warrant for his arrest on war crimes charges.

The court said Putin ‘is allegedly responsible for the crime of unlawful deportation’ of Ukrainian children to the Russian Federation.