Russian soldiers have fired warning shots into the air as around 300 Ukrainian troops marched on the seized Belbek air base in Crimea.
The unarmed Ukrainian servicemen, waving flags, approached the base which has been held by Russian forces.
Around a dozen Russian soldiers warned them not to approach, before firing several shots into the air and reportedly saying they would shoot the Ukrainians if they continued to approach the base.
Warning shots fired by Russian troops in Crimea Russian soldiers confront a Ukrainian officer outside Belbek airbase
The US suspended all military engagements with Russia over its deployment of troops in Crimea. Some 16,000 Russian troops are on the ground in the region - a move that the US said is in a clear violation of international law.
Pentagon spokesman Rear Admiral John Kirby said all military exercises, port visits, bilateral talks and planning conferences between Russia and the US are on hold because of Moscow's actions.
Russian President Vladimir Putin has ordered troops taking part in military exercises close to the Ukrainian border in western Russia to return to their permanent bases on Tuesday.
Russian troops fire warning shots in Crimea The unarmed Ukrainian servicemen had marched on the base
But hundreds of Russian soldiers have surrounded a military base near the Crimean capital Simferopol, preventing Ukrainian soldiers from going in or out.
Russian forces have also seized a border checkpoint on the Ukrainian border between Russia and Crimea, Reuters reported.
Moscow's UN envoy told a stormy meeting of the UN Security Council that Ukraine's ousted leader Viktor Yanukovych had sent a letter to Mr Putin requesting that he use Russia's military to restore law and order in Ukraine.
Churkin, Russia's ambassador to the United Nations, shows a letter to the UN Security Council purportedly from ousted Ukrainian leader Yanukovych to Putin, in New York Vitaly Churkin shows the letter purportedly from Viktor Yanukovych
Russian ambassador Vitaly Churkin, reading from an unofficial translation of the letter he said was dated March 1, said: "The country has plunged into chaos and anarchy.
"The country is in the grip of outright terror and violence driven by the West. People are persecuted on political and language grounds.
Vladimir Putin watches a live firing military exercise President Putin was filmed watching war games in Russia
"In this context, I appeal to the President of Russia Vladimir V Putin to use the armed forces of the Russian Federation to re-establish the rule of law, peace, order, stability and to protect the people of Ukraine."
But US President Barack Obama warned Moscow it would find itself "on the wrong side of history" - and that Russia's deployment of troops in Ukraine violates international law.
He spoke shortly after Mr Putin's aides were forced to deny reports that Russian forces have given the Ukrainian navy until 3am (5am local time) on Tuesday to surrender.
America said any threat by Russia to Ukraine forces would represent a "dangerous escalation" in the crisis, and Moscow would be held responsible.
Moscow has also threatened it could be forced to drop the dollar as a reserve currency and refuse to pay off loans to US banks if the US imposes sanctions over the Ukraine crisis.
Kremlin economic aide Sergei Glazyev said: "We would find a way not just to reduce our dependency on the US to zero but to emerge from those sanctions with great benefits for ourselves.
Ukraine military Ukraine's military might is dwarfed by Russia's
"An attempt to announce sanctions would end in a crash for the financial system of the US, which would cause the end of the domination of the US in the global financial system."
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