By Mick Krever, CNN

American and European action “has possibly prevented a full-scale invasion of Ukraine,” Polish Foreign Minister Radosław Sikorski told CNN’s Christiane Amanpour on Monday.

“America provided leadership on Ukraine,” Sikorski said, countering the common narrative that the West stood weakly in the face of Russian aggression.

U.S. President Barack Obama is headed to Europe this week on a trip that will end by commemorating the 70th anniversary of the D-Day landings at Normandy. His first step, however, will be Poland, which played a key role in the Ukraine crisis.

“We are always glad to see President Obama in Europe,” Sikorski said.

Despite a presidential election, Ukraine remains extremely volatile. Five militants were killed in the volatile eastern Ukrainian city of Luhansk on Monday after they took part in a large coordinated assault on a border guard base, an official at the Ukrainian State Border Guard Service told CNN.

In response to Ukrainian unrest, NATO bolstered its presence near Russia’s border; the U.S. sent 150 paratroopers to Poland.

“We would welcome more troops, both from Europe and from the United States,” Sikorski said.

“We simply think that the level of security across the NATO area should be more or less even. And at the moment, we have NATO bases as legacies of the Cold War, in places where they were useful during the confrontation with the Soviet Union and it doesn't take into account the events of the last quarter of a century. And this should now be addressed.”

He nonetheless expressed hope that Ukraine’s new leader, Petro Poroshenko, can bridge the divide necessary to bring stability.

“Petro Poroshenko comes from the Party of Regions, was a minister under Mr. Yanukovych, and is someone who is credible for the West and should be someone that Russia should be able to do business with.”

“I think they are talking already, and it would be good for the two countries to normalize relations, because they need each other.”

Russian President Putin, he told Amanpour, has perhaps realized that he bit oof more than he could chew in Ukraine.

“I think the costs of annexing Crimea are probably much higher than he had budgeted for. One hears that pension salaries are not being paid, that the banking system is in disarray; the agriculture; the tourist season is lost. And it's costing tens of billions of dollars.”

“Eastern Ukraine is three-and-a-half times bigger, so the cost would be proportionally higher. But it's good when realism wins out.”

So will Russia, Amanpour asked, re-join the path of normalization – joining international treaty and trade organizations – that it was on before the current crisis?

“Only one person knows, and that's President Putin.”