KIEV (AFP) - Western-backed tycoon 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.

Mr 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 stemming a bloody crisis that has shaken the post-Cold War order and redrawn Europe's map.

The 48-year-old candy magnate - dubbed the "chocolate king" - first asked a packed session of parliament to observe a minute of silence for the 100 people killed in three days of a brutal crackdown in Kiev that led to the February ouster of Ukraine's Kremlin-backed regime.

The self-made billionaire then vowed to grant 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 - whose Crimea peninsula was annexed by Russia in March - from fracturing further along ethnic lines.

"I am assuming the presidency in order to preserve and strengthen Ukraine's unity," Mr Poroshenko said in a confident address to parliament that symbolically alternated between Russian and Ukrainian.

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

But Mr Poroshenko also said he would never accept Russia's seizure of Crimea or attempts to divert Ukraine's pro-European course.

US Secretary of State John Kerry voiced hope that Mr Putin's first talks with a Ukrainian leader since the Kiev uprising heralded an easing of the standoff that has also unsettled eastern Europe's ex-Soviet satellite states.

"I hope that in the next few days we can see some steps taken that will reduce the tensions," Mr Kerry told reporters in France.

"I'm confident there are ways forward," he added.

US Vice-President Joe Biden attended Mr Poroshenko's inauguration and the White House said Mr Biden had pledged an additional US$48 million (S$60 million) in assistance aimed at helping the new leader's team protect Ukraine's eastern border and reduce its costly dependence on Russian gas imports.

But separatist commanders whom the West accuses Russia of openly backing dismissed Mr Poroshenko's presidency as illegitimate.

"He is the president of another country," the self-styled prime minister of the "Donetsk People's Republic" told Russia's RIA Novosti news agency.

Separatists in the self-proclaimed "Republic of Lugansk" called on the new Ukrainian president to withdraw his army from the east.

"Negotiations cannot begin until the retreat of the occupying troops from our territory," said the "president" in Lugansk, Mr Valery Bolotov, in a statement to the press.

Mr 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.

Mr 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 endemic corruption that has turned Ukraine into one of Europe's poorest countries and has fed broad public discontent.

He tackled his most urgent assignment Friday by shaking hands with Mr Putin on the sidelines of D-Day commemorations in Normandy 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 recognising him as the legitimate leader of the Ukrainian people.

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

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

Western powers have threatened to slap punishing new sanctions on entire sectors of Russia's economy should Putin fail to demonstrate a more cooperative approach by the end of the month.

And the Kremlin chief appeared to take the first step in that direction Saturday by ordering the Federal Security Service (ex-KGB) to reinforce its protection of the Ukrainian border to stem the flow of Russian arms into the separatist east.

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 rebels who had crossed over into Ukraine from Russia.

Militants on Friday shot down a Ukrainian cargo plane near Slavyansk - an insurgency 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 that three servicemen were killed in the incident and two remained missing.

Mr Putin sounded a surprisingly upbeat note after Friday's brief chat with Mr Poroshenko - a meeting that he grudgingly accepted after personal intervention by German Chancellor Angela Merkel.

"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.

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

Mr Poroshenko brushed aside Mr Putin's pressure Saturday.

"We must sign this agreement by June 27 at the latest," he told a reception of foreign dignitaries after delivering his inauguration address.