U.S. military assets in the Black Sea are on high alert amid reports Ukraine troops traded gunfire Tuesday with pro-Russian militants.

A journalist with Associated Press reported intense gunfire at an airport in eastern Ukraine after the government sent in army troops to retake control from militiamen loyal to Russian President Vladimir Putin.

The mayor of Kramatorsk told the Associated Press that Ukrainian troops have now occupied the military airport and are blocking its entrance.

The action comes a day after the revelation a Russian fighter jetted buzzed a U.S.S. destroyer multiple times over the Black Sea.

Russia declared Ukraine on the brink of civil war, and Kiev said an “anti-terrorist operation” against pro-Moscow separatists was under way, with troops and armored personnel carriers seen near a flashpoint eastern town.

Twenty-four hours after an Ukrainian ultimatum expired for the rebels to lay down their arms, witnesses however saw no signs yet that Ukraine’s forces were about to storm state buildings in the Russian-speaking east that armed militants have occupied.

Interim President Oleksander Turchinov insisted the operation had started in the eastern Donetsk region, although it would happen in stages and “in a considered way.”

“It will be conducted step by step, responsibly, cautiously,” he said. “The aim of these actions is to protect the citizens of Ukraine, to stop terror, to stop criminality, to stop attempts to tear Ukraine to pieces.”

The pro-Russian mayor of Slaviansk claimed Tuesday that a Ukrainian military convoy including troops and armored vehicles had now surrounded the town, Russian state news agency ITAR-Tass reported.

“If they try to move in, we will have to stop them,” the acting mayor said, according to the news agency.

Amidst the deepest East-West crisis since the Cold War, Mr. Putin and President Obama spoke by phone Monday and called on each other to do all in their power to avoid further bloodshed.

The standoff has raised fears that Moscow might turn off gas supplies to Kiev, disrupting flows to the European Union. Russian exporter Gazprom promised it would remain a reliable supplier to the EU, but German energy company RWE began deliveries to Ukraine on Tuesday - reversing the usual east-west flow in one central European pipeline.

— This article is based in part on wire service reports.

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