Kiev (AFP) - Ukraine's new pro-Western leader enters a defining week Sunday seeking to head off a Russian gas cut and secure US President Barack Obama's backing with his country threatened by civil war.

Confectionery tycoon Petro Poroshenko will also attempt to arrange the first meeting by a Ukrainian leader with Russian President Vladimir Putin since the February ouster of a pro-Kremlin regime in Kiev sparked the worst East-West crisis since the Cold War.

The 48-year-old will cap off the pivotal week with his inauguration Saturday as the fifth president of Ukraine after a convincing May 25 election win handed him a mandate to resolve a separatist insurgency threatening the very survival of the ex-Soviet state.

"These meetings will be crucial," said Kiev-based political analyst Volodymyr Fesenko.

"They will help establish direct relations and introduce Petro Poroshenko to world leaders -- first and foremost, Barack Obama."

- Russian gas threat -

Ukraine's third "gas war" with Russia in less than a decade erupted when Moscow -- stunned by the sudden ouster of an ally who had just rejected an EU alliance that the Kremlin greatly feared -- nearly doubled the price it charges its neighbour for the fuel.

Kiev accused Moscow of "economic aggression" and refused to cover a bill that Russia puts at $5.17 billion (3.79 billion euros).

Russian gas transits through Ukraine to supply about 15 percent of European needs and a top EU envoy is now urgently seeking a compromise that could save 18 member states from seeing their deliveries start dwindling Wednesday.

A final round of talks has been set for Monday in Brussels after Ukraine's Naftogaz state energy firm -- bowing to both EU and Kremlin pressure -- transferred a $786 million payment to its Russian counterpart Gazprom to keep the talks alive.

Gazprom now says it is willing to discuss a lower price and analysts believe that a compromise is in sight because Russia would prefer to avoid complicating its relations with Europe still further.

Ukrainian Prime Minister Arseniy Yatsenyuk added to the general cheer surrounding Monday's meeting by promising to settle Russia's entire bill using a part of a new Western financial rescue package.

"We will pay our bills," Yatsenyuk told German public television in an interview aired Sunday.

"First we have to sign a (new price) contract. And then, 10 days later, Ukraine will pay."

But he also warned that Ukraine would follow the example of other Gazprom clients who had won their own price disputes through the courts by filing for arbitration in Stockholm by end of day Monday if no agreement was reached.

"If Russia does not want a contract, we will meet in court in Stockholm," Yatsenyuk said.

- Obama handshake -

Poroshenko will shake hands with Obama on Wednesday in Warsaw wary of an address last week in which the White House chief put American diplomacy above military might in confronting threats such as that of Russia's expansion.

"Just because we have the best hammer does not mean that every problem is a nail," Obama told US Military Academy graduates.

That message was greeted with awkward silence by former Soviet satellites in eastern Europe that have been clamouring for firmer US protection since Russia's seizure of Ukraine's strategic Black Sea peninsula of Crimea in March.

Washington is sending Assistant Secretary of Defence Derek Chollet for meetings in Kiev on Monday aimed at reassuring Poroshenko and showing Putin the strength of the US commitment to Ukraine.

"I am certain that as soon as president-elect Poroshenko is sworn into office, we will begin discussions about our future cooperation on security and defence," US Ambassador Geoffrey Pyatt told Kiev's Dzerkalo Tyzhnia newspaper.

Poroshenko for his part promised to seek a "new security alliance with the United States and Europe" that could protect Ukraine without its outright membership in the Cold War-era NATO bloc that Russia views with hostility and mistrust.

- Elusive Putin meeting -

Putin spelt out the threat of an outright invasion of Ukraine when he sought and won parliament's authorisation on March 1 to use any means necessary to "protect" ethnic Russians living across the border.

But the drumbeats of war began fading last month when Putin surprised many by suddenly softening his tone.

Story continues