WARSAW (AFP) ― U.S. President Barack Obama launched a European tour on Tuesday shaped by the escalating separatist insurgency in eastern Ukraine and worries among former Soviet satellites about Russia’s new expansionist threat.

Obama landed in Warsaw for celebrations of the 25th anniversary of Poland’s first free elections that put both the country and the rest of eastern Europe on a path out of Moscow’s orbit and toward democracy and growing economic prosperity.

But his first big meeting will come Wednesday when he meets Ukraine’s embattled President-elect Petro Poroshenko with the ex-Soviet state threatened by civil war and its new pro-Western leadership grasping for protection from Washington.

The seven-week pro-Russian insurgency in Ukraine’s eastern rust belt grew only more violent after Poroshenko swept to power in a May 25 presidential ballot on a promise to quickly end fighting and save the nation of 46 million from economic collapse.

Hundreds of separatist gunmen staged one of their biggest offensives to date on Monday by attacking a Ukrainian border guard service camp in the Russian border region of Lugansk.

Ukraine’s military reported suffering no fatalities and killing five rebels in a day-long battle that saw insurgents pelt the camp with mortar fire and deploy snipers on rooftops surrounding the base.

But Lugansk’s self-declared “prime minister” Vasyl Nikitin told AFP that at least three civilians and the separatist administration’s top health official had died in the violence.

Ukraine’s interior minister on Tuesday urged civilians across parts of the neighbouring coal mining region of Donetsk to stay indoors “in order to avoid risking their lives.”

Arsen Avakov wrote on Facebook that a “very intense gunfight” was raging on Tuesday morning in the small Donetsk region town of Semenivka.

Washington’s commitment to Ukraine will be reinforced when U.S. Vice President Joe Biden travels to Kiev on Saturday to attend Poroshenko’s swearing in as the fifth post-Soviet president of Ukraine.

The visit is meant to underscore the U.S. position that the people of Ukraine ― and not Moscow ― should decide their destiny and overcome the cultural differences now tearing apart the vast country’s Russified east and more nationalist west.

Kiev has not yet invited any Moscow official to the inauguration and Russian President Vladimir Putin is yet to formally recognise the result of an election that saw rebels disrupt voting across swathes of the east.

Ukraine and its eastern European allies such as Poland have been pushing Washington and EU leaders to unleash painful economic sanctions against entire sectors of Russia’s economy for the Kremlin’s perceived support of the rebels.

Obama‘s tour takes in the Group of Seven summit in Brussels on Thursday that symbolically replaces a Group of Eight meeting that Putin was due to host in Sochi but which world leaders decided to boycott.

But Obama’s most sensitive part of his swing comes on Friday when he attends that 70th anniversary commemoration of D-Day in Normandy to which Putin was invited as well.

The U.S. leader has spent months trying to isolate his rival and punish the Kremlin inner circle with sanctions over Ukraine.

Both the Kremlin and White House say no one-on-one meeting between Obama and Putin is being planned.

But senior White House aides have not ruled out an informal encounter -- which would be the first for the rivals since the Ukraine crisis mushroomed into the worst East-West standoff since the Cold War.

Ukraine’s Poroshenko -- a 48-year-old chocolate baron who once enjoyed good working relations with Moscow while serving as foreign minister -- won a surprise reprieve on Monday when Russia’s state gas firm Gazprom delayed a threatened cut in fuel shipments that would also impact large portions of Europe.

The decision to give Ukraine an extra week to make its payments came after Kiev accused Moscow of “economic aggression” for its decision to cancel the gas price discounts it awarded pro-Kremlin leader Viktor Yanukovych prior to his February fall.

Moscow had threatened to halt all shipments to Ukraine -- a vital gas transit nation now seeking a closer alliance with the West -- from Tuesday in a repetition of interruptions that also hurt swathes of Europe in 2006 and 2009.

Ukraine now has until June 9 to start covering its debts -- a period that Russia has promised to use to help reach a long-term price compromise.