WASHINGTON, May 26 - 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, AFP reported.

"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."

Oleksandr Turchynov, the head of Ukraine's National Security Council, said last week that Russia has become an increased threat since annexing the Black Sea peninsula of Crimea and increasing its military presence there.

Ukraine has repeatedly raised alarms about what it sees as Russia's aggressive military posture. It says Moscow has actively supplied separatists in east Ukraine with arms and manpower and that it routinely bolsters offensive capabilities in western Russia.

President Petro Poroshenko's government is concerned that Russia is making concerted efforts to move its nuclear capabilities to Crimea, which was absorbed by Moscow in 2014 following a referendum almost universally rejected by the international community.

Poroshenko said at a press conference in Kiev with Slovakian counterpart Andrej Kiska that Russia has between 4,000 and 14,000 soldiers stationed in Ukraine at any given
time.

"This is not just Ukrainian information. This is intelligence from NATO countries and other sources," Poroshenko said. (afp/ez)