Ukraine (AP) - Vladimir Putin ordered tens of thousands of Russian troops participating in military exercises near Ukraine's border to return to their bases as U.S. Secretary of State John Kerry was on his way to Kiev. Tensions remained high in the strategic Ukrainian peninsula of Crimea with troops loyal to Moscow fired warning shots at protesting Ukrainian soldiers.It was not clear if Putin's move was an attempt to heed the West's call to de-escalate the crisis that has put Ukraine's future on the line.It came as Kerry was on his way to Kiev to meet with the new Ukrainian leadership that deposed a pro-Russian president, and has accused Moscow of a military invasion. The Kremlin, which does not recognize the new Ukrainian leadership, insists it made the move in order to protest millions of Russians living there.On Tuesday, pro-Russian troops who had taken control of the Belbek air base in the Crimea region fired warning shots into the air as around 300 Ukrainian soldiers, who previously manned the airfield, demanded their jobs back.About a dozen Russian soldiers at the base warned the Ukrainians, who were marching unarmed, not to approach. They fired several warning shots into the air and said they would shoot the Ukrainians if they continued to march toward them.The shots were apparently the first fired since pro-Russian troops estimated by Ukrainian authorities to be 16,000 strong tightened their grip on the Crimea Peninsula over the weekend.There was no fighting elsewhere in Crimea early on Tuesday. A supposed Russian ultimatum for two Ukrainian warships to surrender or be seized passed without action from either side, as the two ships remained anchored in the Crimean port of Sevastopol. Russian Defense Ministry spokesman Vladimir Anikin said late Monday that no ultimatum had been issued.Early on Tuesday, the Kremlin said Putin ordered troops participating in military exercises in western Russia near the Ukraine border to return to their permanent bases. The order was issued almost a week after Russia began massive exercises involving most military units in western Russia, stoking fears that the Kremlin might use the troops to seize territory in pro-Russian areas of eastern Ukraine.In Brussels, meanwhile, the ambassadors of NATO's 28 member nations will hold a second emergency meeting on Ukraine on Tuesday after Poland, which borders both Russia and Ukraine, invoked an article calling for consultations when a nation sees its "territorial integrity, political independence or security threatened," the alliance said in a statement.President Barack Obama has said that Russia is "on the wrong side of history" in Ukraine and its actions violate international law.Ivan Sekretarev in Sevastopol contributed to this report.