Obama said that Ukraine could be a vibrant, thriving democracy if the US and the international community stood behind it.

Speaking after his first meeting with Ukraine's president-elect, Petro Poroshenko, on Wednesday in Warsaw, Obama said he would offer US support for the Ukrainian economy to help ensure it could get through the winter if Russian gas supplies were cut due to an ongoing payment row.

Talks continued in Berlin on Wednesday between Ukraine's state-owned Naftogaz and Russia's Gazprom to settle the dispute ahead of the June 9 deadline for Ukraine to pay its gas debts.

Obama praised Poroshenko for reaching out to the country's restive eastern regions. He said that other nations should support Ukraine and the new government by training its military and police to deal with challenges and ensuring that Russia no longer supported separatists in Ukraine.

US Vice President Joe Biden travels to Kyiv on Saturday to attend Poroshenko's swearing-in as the country's fifth post-Soviet president. Russia's President Vladimir Putin was not invited as he has not formally recognized the May 25 vote which brought Poroshenko to power.

US in Europe

Obama was in Poland to mark the 25th anniversary of the end of Soviet rule. He made a speech on Wednesday in the open air in Warsaw in front of the Royal Castle, a symbol of Polish indepence: "We stand together because we believe that people and nations have the right to determine their own destiny - that includes the people of Ukraine," Obama said.

"Our free nations will stand united so that further Russian provocations will only mean more isolation and costs for Russia," Obama said. "Because after investing so much blood and treasure to bring Europe together, we refuse to allow the dark tactics of the 20th century to define the 21st."

Obama had said on Tuesday that US commitment to eastern European security was absolute, after inspecting a joint unit of Polish and US F-16 pilots. He also announced a $1-billion (734-million-euro) plan to boost military deployments in Europe as a response to Russian actions in Ukraine.

The US President encouraged Putin to accept Poroshenko's invitation to hold talks when both leaders attend Friday's commemoration of the D-Day landings in Normandy, France.

Putin is due to hold one-on-one talks with German Chancellor Angela Merkel, Britain's David Cameron and French President Francois Hollande on the sidelines of the 70th anniversary ceremonies.

Obama has since arrived in Brussels from Poland for G7 summit to discuss foreign policy, including Russia and EU assistance to Ukraine, as well as Syria, Afghanistan, Mali, the Central African Republic and North Korea. For the first time in 17 years, Russia will not be part of the group. This year's G8 summit was formerly scheduled to be held in Sochi, with Russia the holder of the rotating G8 presidency in 2014.

The decision to suspend Russia from the G8 and relabel it the G7 was taken by the group's other members - the United States, Germany, France, Britain, Canada, Japan and Italy - in March, after Moscow annexed Crimea.

jm/msh (Reuters, AP)