Petro Poroshenko has taken oath as President of Ukraine firmly reiterating that the Crimean region, annexed to Russia, belongs to Ukraine.

During his swearing-in ceremony at the parliament in Kiev, the 48-year-old confectionery magnate said: "Crimea is, was and always will be Ukrainian soil."

The secession of Crimea from Ukraine, followed by its annexation to Russia, has been the focal point of the latest geopolitical crisis between Moscow and the Western powers, with Ukraine caught in the middle.

Poroshenko, in his remarks at the inauguration, called for peace and unity, but emphasised that he will not compromise on Ukraine's territorial integrity.

"I pledge with all my strength to protect the sovereignty and independence of Ukraine.

"I will make unity happen. I don't want war. I don't want revenge, despite the huge sacrifice of the Ukrainian people."

He exhorted the pro-Russian separatists in Ukraine's eastern region to lay down their weapons and vowed to punish those who were behind attacks on Ukrainian people.

The Russian ambassador to Kiev was among scores of dignitaries who attended the ceremony.

Shortly after the ceremony, Poroshenko signed the first decree assuming full powers of the country's armed forces.

"In connection with taking the oath of the President of Ukraine on June 7, 2014, in accordance with paragraph 17 Part 1 Article 106 of the Constitution of Ukraine I assume this day the powers of the Supreme Commander-in-Chief of the Armed Forces of Ukraine," the document of the presidential website reads.

He also asserted that the Ukrainian language will remain the only official language although other languages including Russian will be allowed to exist without too many restrictions.