Western-backed businessman Petro Poroshenko vowed Saturday to avert civil war and mend ties with Russia after being sworn in as Ukraine's fifth post-Soviet president with the nation facing disintegration and economic collapse.

Poroshenko took the oath of office one day after holding his first meeting with Russian President Vladimir Putin since a May 25 election victory entrusted him with taming a bloody crisis that has shaken the post-Cold War order and redrawn Europe's map.

The 48-year-old magnate, dubbed the "chocolate king" for his popular brand of sweets, first asked a packed session of parliament to pay a minute of silence for the 100 people killed in three days of carnage in Kiev that led to the February ouster of Ukraine's Kremlin-backed regime.

The self-made billionaire then vowed to give an amnesty to any insurgents who had "no blood on their hands" as the first step in a peace initiative designed to save the nation of 46 million, which saw its Crimea peninsula annexed by Russia in March, from splitting further along ethnic lines.

"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.

"The citizens of Ukraine will never feel the blessing of peace and security until we resolve our relations with Russia."

But Proshenko also added that he would never accept Russia's seizure of Crimea or attempts to divert his pro-European course.

"Crimea will remain a part of Ukraine," Poroshenko said firmly.

"Ukraine now returns to its natural European condition that so many generations have longed for."

Saturday's solemn ceremony was attended by U.S. Vice President Joe Biden and European Council President Herman Van Rompuy along with more than 20 other dignitaries from countries that back Kiev's new Westward drive.

"All neighbours... need to respect (Ukraine's) sovereign choices, including stronger ties with the European Union and its territorial integrity," Van Rompuy said in a clear reference to Russia.

But Moscow was only represented by its acting ambassador to Kiev -- a telling sign of how far relations between the two neighbours have slipped since the February revolt.

Poroshenko is one of Ukraine's more experienced politicians who held senior cabinet posts under both the Western-leaning government that followed Ukraine's 2004 Orange Revolution and the Moscow-friendly leadership of ousted president Viktor Yanukovych.

That pragmatic approach has instilled hope among many Ukrainians that he will be able to resolve an eight-week secessionist drive by pro-Russian militants in the eastern rust belt that has claimed 200 lives and grown even more violent since his election.

Poroshenko -- who has vowed to give up direct ownership of his holdings to avoid a conflict of interest -- must also address a two-year recession and tackle endemic corruption that has turned Ukraine into one of Europe's poorest countries and has fed broad public discontent.

A step in that direction may have been taken in Normandy Friday when he shook hands with Putin on the sidelines of D-Day commemorations that were haunted by the spectre of an outright civil war breaking out on the European Union's eastern edge.

Moscow had previously said it was ready to work with the new president but stopped short of explicitly recognizxing him as the legitimate leader of the Ukrainian people.

U.S. President Barack Obama, who met Putin for 10 minutes on Friday despite earlier efforts to isolate the hardline Kremlin chief, told NBC Nightly News that Russia had to recognize Poroshenko as legitimate if it wanted to resolve the flaring conflict.

"Mr Putin should be working directly with Mr Poroshenko and the government of Ukraine to try to resolve differences between the two countries," Obama stressed.

Russia also needs "to stop financing and arming separatists who have been wreaking havoc in the eastern part of the country," Obama added.

Mounting tensions in the rebel regions have seen Kiev concede that it was losing control of three border posts that were being routinely attacked by the rebels.

Insurgents on Friday shot down a Ukrainian military cargo plane near Slavyansk,  a rebel stronghold where many of the 120,000 residents have been forced to spend nights in basements because of the ceaseless fighting.

A military spokesman said Saturday that "some crew members" managed to evacuate in time. But he gave no casualty figure.

Putin sounded a surprisingly upbeat note after his meeting with Poroshenko.

"I cannot but welcome the position of Poroshenko on the necessity to end the bloodletting immediately in the east of Ukraine," he told reporters in France.

"I cannot say for sure how that can be implemented in practical terms, but overall it seemed to be to be the right approach," Putin said.

"Ukraine must demonstrate its good will. The repressive operation must be stopped," said Putin.

"I hope that will happen, and if that happens, the conditions would be there for the development of our relations in other areas, including economic."

But Putin also warned that Russia would have no choice but to slap trade restrictions on Ukraine should it proceed with plans to sign an historic economic treaty with the European Union in the coming weeks.