Ukraine said Russia was building up armored vehicles on its side of a narrow stretch of water near the Ukrainian region of Crimea after President Vladimir Putin said he had the right to enter into his neighbor’s territory.

Ukraine said Russia was building up armored vehicles on its side of a narrow stretch of water near the Ukrainian region of Crimea after President Vladimir Putin said he had the right to enter into his neighbor's territory, prompting a sell-off in Russian assets, World Bulletin website reported.

Ukraine mobilized for war on Sunday and Washington threatened to isolate Russia economically after Putin's declaration, provoking Moscow's biggest confrontation with the West since the Cold War.

The Russian central bank raised its key lending rate 1.5 percentage points after the rouble fell 2.5 percent to an all-time against the dollar at the opening of exchange trading on Monday, while the MICEX index of Moscow stocks tumbled 10 percent to 1,294 points. Russian gas monopoly Gazprom , which supplies Europe through Ukraine, was down more than 13 percent.

Ukraine's Prime Minister Arseny Yatseniuk, head of a pro-Western government that took power when former president Viktor Yanukovich, a Russian ally, left capital on Feb. 21 after three months of street protests against his rule, said Putin had effectively declared war on his country.

A Ukrainian border guard spokesman said on Monday that Russian ships had been moving in and around the Crimean port city of Sevastopol, where the Russian Black Sea Fleet has a base, and that Russian forces had blocked mobile telephone services in some parts of Crimea.

He said the build-up of Russian armor was near a ferry port on the Russian side of what is known as the Kerch Strait, which separates the eastern edge of the Crimea peninsula and the western edge of the Taman Peninsula.

The strait is 4.5 km (2.8 miles) wide at its narrowest point and up to 18 meters (59 feet) deep.

The border guard spokesman did not say how many armored vehicles had gathered in Russian territory, opposite the city of Kerch on the Ukrainian side of the strait.

There was no immediate comment from the Russian Defence Ministry.