U.S. President Barack Obama has met privately in Warsaw with Ukraine's President-elect Petro Poroshenko in a show of support for Kyiv amid a separatist insurgency in the east.
The meeting on June 4 came a day after Obama pledged to spend $1 billion to send more U.S. military to Europe on a temporary basis.
Obama is also due to deliver a major speech to highlight the 25th anniversary of Poland's first free elections.
He is expected to say that Poland's experience can serve as an example for Ukraine.
NATO accuses Russia of being behind the revolt in eastern Ukraine, a charge Moscow denies, although it asserts the right to protect Russian-speakers in the region.
Speaking after talks with Polish President Bronislaw Komorowski in the Polish capital on June 3, Obama denounced the separatists in eastern Ukraine as "a bunch of masked thugs creating chaos."
He said a mechanism was needed to return law and order to these areas and "this is where Russian influence can be extraordinarily important."
Meanwhile, NATO Secretary-General Anders Fogh Rasmussen announced on June 3 at a NATO meeting in Brussels that the military alliance will finalize by the end of June a package aimed at modernizing and reforming Ukraine’s armed forces.
Rasmussen said the defense ministers agreed to bolster the alliance's defense capabilities, in part by stepping up its activities across the European continent.
"We agreed that we will continue to reinforce NATO's collective defense with more air and sea patrols and more exercises and training from the Baltic Sea to the Black Sea and the Mediterranean," he said.
At the same NATO meeting, U.S. Defense Secretary Chuck Hagel urged NATO allies to raise their defense budgets due to the Ukraine crisis.
He also called on allies to review how their militaries were trained and equipped to meet new security challenges, arguing NATO should expect Russia to continue to test its resolve.
As part of measures to improve NATO's readiness, Germany, Denmark, and Poland announced plans on June 3 to boost their joint NATO corps in the Polish city of Stettin, where around 180 military staff currently plan operations and exercises.
According to the dpa news agency, acting Defense Minister Mykhaylo Koval said that Ukraine had the "necessary capabilities to defend the country," although it did have some shortfalls regarding "air defense systems, intelligence and communication equipment."
Inside Ukraine, fighting was reported for a second day in and around the rebel-held city of Slovyansk in the east.
A Ukrainian military spokesman said some 300 separatists had been either killed or wounded, a figure that could not be independently verified.
With reporting by dpa and Reuters