New Ukrainian President Petro Poroshenko waves as he leaves his inauguration ceremony in Kiev June 7, 2014. Photo: Konstantin Grishin.

Kiev - Petro Poroshenko was sworn in as Ukraine's fifth post-Soviet president on Saturday, vowing to maintain the unity of his country amid a continuing crisis with Russia.

Poroshenko, a 48-year-old billionaire who won the presidential election on May 25 with 54.7 percent of the vote, took the oath in the Ukrainian parliament in Kiev.

“I am assuming the presidency in order to preserve and strengthen Ukraine's unity,” Poroshenko said in an address that alternated between Ukrainian and Russian.

Speaking to an audience that included US Vice President Joseph Biden, he promised the residents of the Donbass region, which is largely in rebel hands, that he would decentralise power and guarantee the free use of the Russian language.

But he also said that there would be no compromise with Russia on his pro-European stance and the status of the Crimean peninsula.

“Crimea has been and will remain Ukrainian,” he said. “I put that clearly to the Russian leader in Normandy,” he added, referring to his meeting with President Vladimir Putin at D-Day commemorations on Friday.