The border guards say Russian servicemen are in control of the Crimean side of the narrow channel and that Russian armored vehicles have been sighted on the Russian side.

"The Kerch Strait is blocked by two Russian ships - from the north and from the south," Pavel Shishurin, the deputy head of the border guards, told reporters.

The Russian military has not confirmed his comments.

The Russian Navy vessels had the Ukrainian ships Ternopil and Slavutych boxed inside the Black Sea port of Sevastopol and were refusing to let them leave, the New York Daily News reported.

Accusing the Russians of “piracy,” Ukraine’s acting President Oleksandr Turchynov said the captains and crews were “ready to defend their ships.”

“Total nonsense,” said Russian Defense Ministry spokesman Vladimir Anikin, who insisted no such ultimatum has been delivered to Ukraine’s government in Kiev.

“We’re interested in keeping friendly relations with the people of Ukraine and in preserving stability.”

The US President Barack Obama said he is taking the threat seriously and warned that seizing the ships would be a “dangerous escalation.”

“Over time, this will be a costly proposition for Russia,” Obama said, after dispatching Secretary of State Kerry to Kiev.

But the European Union wasn’t as eager to take on the Russians, largely because it gets a quarter of its oil and gas from them. “There’s the condemnation of Russian intervention and then there’s the need for mediation, for dialogue,” said French Foreign Minister Laurent Fabius.

Tensions between Kiev and Moscow erupted after Ukraine’s parliament voted to remove pro-Russia President ViKtor Yanukovych from power and release pro-West former prime minister Yulia Tymoshenko from prison on February 21.

NTJ/BA