Two Russian warships crossed Turkey's Bosphorus Strait early Tuesday headed towards the Black Sea, Turkish media reported.

The reported ship movements came with Russia and the West locked in a standoff over Ukraine's Crimean peninsula on the Black Sea.

Russian Foreign Minister Sergei Lavrov said Tuesday that threats of sanctions against Moscow are counter-productive and will not change its position on the Ukraine crisis.

"Our position is honest and... will not change," Lavrov told a news conference in Tunisia, where he was on an official visit.

President Vladimir Putin denied on Tuesday that Russian troops were operating in the Ukrainian Black Sea region of Crimea, while sending Moscow's forces to Ukraine would be an entirely legitimate move but also a last resort.

U.S. President Barack Obama, however, said Putin's rationale for his incursion into Crimea was not "fooling anybody" and said Russian "meddling" would push states away from Moscow.

Ukraine Prime Minister Arseniy Yatsenyuk will meet EU leaders Thursday ahead of an emergency summit the same day on the crisis in his country, EU President Herman Van Rompuy said.

EU leaders "will discuss situation in Ukraine with PM Yatsenyuk in Brussels Thursday prior to extraordinary summit," Van Rompuy said in a Tweeted message Tuesday.

Russia could reduce to zero its economic dependency on the United States if Washington agreed sanctions against Moscow over Ukraine, a Kremlin aide said on Tuesday, warning that the American financial system faced a "crash" if this happened.

"We would find a way not just to reduce our dependency on the United States to zero but to emerge from those sanctions with great benefits for ourselves," said Kremlin economic aide Sergei Glazyev.

Russian President Vladimir Putin on Tuesday told troops to return to their permanent bases after calling a snap drill to check their battle-readiness last week.

"The commander-in-chief President Vladimir Putin gave the order to the troops and units taking part in military exercises to return to their permanent bases," Putin's spokesman Dmitry Peskov told Russian news agencies.

The United States suspended military cooperation with Russia due to its military intervention in Ukraine, as Secretary of State John Kerry prepared to visit Kiev Tuesday amid a deepening crisis.

"We have, in light of recent events in Ukraine, put on hold all military-to-military engagements between the United States and Russia," Pentagon spokesman Rear Admiral John Kirby said in a statement Monday.

The European Union warned on Monday that EU-Russia ties were at risk if Moscow failed to de-escalate tensions in Ukraine ahead of a U.N. Security Council meeting on the crisis.

"In the absence of de-escalating steps by Russia the EU shall decide about consequences for bilateral relations of EU and Russia," said a statement obtained by Agence France Presse that was agreed by the bloc's 28 foreign ministers at hastily-convened crisis talks on Ukraine.

Russian troops and military planes were flowing into Crimea on Monday in violation of accords between the two countries, Ukrainian border guards said, as Moscow reportedly warned Ukrainian forces in Crimea to surrender or face a confrontation.

The Interfax news agency quoted a Ukrainian defense ministry source as saying that “the Russian fleet has given Ukrainian forces in Crimea a 12-hour ultimatum to surrender or face a storm."

The U.S. envoy and head of the Organization for Security and Cooperation in Europe said on Monday that the OSCE should "immediately" send an observer mission to Ukraine, as the pan-European body met in Vienna on Monday.

"We call today for OSCE observers to be sent immediately to Ukraine," ambassador Daniel Baer said in a speech, released on Monday, to the Organization for Security and Co-operation in Europe.