Ljubljana - Prime Minister Robert Golob has strongly condemned Russia's annexation of Ukraine's Donetsk, Lugansk, Zaporizhzhia and Kherson regions, saying it contravenes the fundamental principles of the Charter of the United Nations and international law.

"Slovenia strongly condemns Russia's unlawful annexation of the Ukrainian regions of Donetsk, Lugansk, Zaporizhzhia and Kherson, which is contrary to the fundamental principles of the UN Charter and international law," he tweeted on Friday, adding the united and strong support of the international community for Ukraine was crucial.

Today's formal annexation of the four Ukrainian regions to Russia has already been condemned in a joint statement by EU leaders, who pledged never to recognise the illegal annexation. The same response came from the White House, which also announced "severe" new sanctions against Russia.

Russian President Vladimir Putin formally annexed the four Ukrainian territories, which are controlled by the Russian army, by signing relevant documents in Moscow, after they held referendums on joining Russia.