Crimea's night sky was illuminated by fireworks as thousands of people celebrated the breakaway Ukraine province's formal union with Russia.

Russian President Vladimir Putin ordered festivities in Moscow and Simferopol after the upper house of Russian parliament ratified a treaty of Crimea's accession.

Crimean Prime Minister Sergei Aksyonov addressed cheering crowds in Simferopol saying the creation of the new Crimea was "fair and we deserved it."

"I am sure that we all together live in a new Crimea happily, we will live the way our fathers and grandfathers wanted us to live as we dreamed. This is fair and we deserved it. Happy holiday, dear friends," Aksyonov told a large gathering.

Thanking the Crimean security personnel for their bravery, Aksyonov said: "Thanks to you, we have obtained confidence and courage. I would like to thank Russian President Vladimir Putin and all Russians who stood by our side backing us up morally and physically when we needed that."

Festive fireworks also lit up Moscow skies at the same time, marking Crimea's accession to Russia.

Meanwhile, a UN team arrived in Crimea to monitor the human rights situation in the region amid intensifying tensions.

UN Assistant Secretary General for Human Rights Ivan Simonovic and his team arrived in Simferopol and held talks with Aksyonov and Crimea's Parliament Speaker Vladimir Konstantinov.

The delegation also met representatives of the ombudsperson, civil society, journalists and regional organisations, UN Secretary-General's deputy spokesperson Farhan Haq told a press conference.