U.S. President Barack Obama accused Russia of toughening its battlefield stance.
"We had a chance to discuss the situation in Ukraine and the increasingly aggressive posture that Russia has taken," Obama said, after Oval Office talks with Jens Stoltenberg.
Both men stressed the need for parties to respect a largely-ignored February ceasefire agreement, which Stoltenberg said could still be a "path to peace."
The deal, between European powers, Russia and Ukraine was aimed at ending a 13-month war that has claimed nearly 6,300 lives and left well over a million people homeless.
In recent days there has been particularly heavy fighting between Ukrainian forces and pro-Russian separatists near the strategic port city of Mariupol.
At the same time there has been an uptick in tensions as Russia aircraft and submarines have made provocative forays into European airspace and waters.
Obama said this was a "challenging and important time for NATO."