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 invade his neighbor.
Russian ships had been moving in and around the Crimean port city of Sevastopol, where the Russian Black Sea fleet has a base, and Russian forces had blocked cellphone services in some parts of Crimea, a Ukrainian border guard spokesman said on Monday.
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 2.8 miles wide at its narrowest point and up to 59 feet deep.
"There are armored vehicles on the other side of the strait. We can't predict whether or not they will put any vehicles on the ferry," the spokesman said by telephone.
Russian President Vladimir Putin seized Ukraine's Crimean Peninsula over the weekend, which has created the greatest confrontation between Russia and the West since the collapse of the Soviet Union in 1991.