US President Barack Obama accused Russia of toughening its battlefield stance Tuesday, as he met with the NATO secretary general to discuss the rolling crisis in Ukraine.

"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 cease-fire 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.

In recent days there has been particularly heavy fighting between Ukrainian forces and the rebels near the strategic port city of Mariupol.

At the same time there has been a rise in ­tensions as Russia aircraft and submarines have made forays into European airspace and ­waters.

Obama said this was a "challenging and important time for NATO."