Ukrainian President Petro Poroshenko expressed confidence that President-elect Trump will defend Ukraine and lead NATO against Russian President Vladimir Putin's aggression.

"President Trump has confirmed that he is sticking on the obligations of the United States," Poroshenko told Reuters.

Poroshenko's comments preface a diplomatic push that he'll be making in person with Trump about maintaining sanctions on Russia until Putin cuts support for separatists fighting in eastern Ukraine, and reverses the annexation of Crimea. Trump's policy toward Ukraine has been a source of anxious speculation in Congress and Europe, given the president-elect's stated desire to forge a stronger relationship with Putin and his criticisms of NATO.

"It is extremely important the transatlantic union should be kept, and this is by the way a very important demonstration of the role of the United States as a global leader," Poroshenko said during the interview, which took place on the sidelines of the World Economic Forum. "America should be great again."

A bipartisan group of senators has introduced legislation calling for mandatory sanctions retaliating against Russian cyberattacks, which would ban "any action ... that recognizes or implies any recognition" of Russia's control of Crimea. Secretary of State nominee Rex Tillerson expressed reservations about having the sanctions be mandatory, rather than providing Trump with the customary authority to waive them when necessary for national security reasons.

But Tillerson also denounced Russia's annexation of Crimea and faulted Obama for failing to deter further attempts to take territory in eastern Ukraine. "If Russia acts with force — the taking of Crimea was an act of force, they didn't just volunteer themselves — so it required a proportional show of force to indicate to Russia that there will be no more taking of territory," Tillerson said.

Like Defense Secretary nominee James Mattis, Tillerson also promised that the United States would honor NATO obligations, which Poroshenko also welcomed.

"I think that the statement of the new candidates — starting from the new candidate for the State secretary and the new candidate for the secretary of defense — are very promising," he said Wednesday. "With that situation, I think it does give us a lot more optimism for the future."