Russia has started a build-up of armoured vehicles on the Russian side of a narrow stretch of water between Russia and the Ukrainian region of Crimea, Ukrainian border guards said.
A border guard spokesman also told the Reuters news agency 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 armour 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 metres (59 feet) deep.
"There are armoured 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.
The border guard spokesman did not say how many armoured 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.
The Kerch Strait also connects the Black Sea and the Sea of Azov.
Russian forces have taken control of Crimea, which has an ethnic Russian majority, and Ukraine has ordered a military mobilisation as well as putting its forces on combat alert.
Russia's upper house of parliament has authorised President Vladimir Putin to deploy troops in Ukraine to defend Russian speakers in Ukraine who are said by Moscow to be under threat.
With Russian forces in command of Crimea, the focus is shifting to eastern swaths of Ukraine, where most ethnic Ukrainians speak Russian as a native language.
Those areas saw more demonstrations on Sunday after violent protests on Saturday, and pro-Moscow activists hoisted flags for a second day at government buildings and called for Russia to defend them.
Russia has staged war games with 150,000 troops along the land border, but they have so far not crossed. Kiev said Russia had sent hundreds of its citizens across the border to stage the protests.
Ukraine's security council ordered the general staff to immediately put all armed forces on highest alert. But Kiev's small and under-equipped military is seen as no match for Russia's superpower might.
The defence ministry was ordered to stage a call-up of reserves, meaning theoretically all men up to 40 in a country with universal male conscription, although Ukraine would struggle to find extra guns or uniforms for many of them.
|US, EU can put economic pressure on Russia
Washington on Sunday threatened to isolate Russia economically after Putin declared he had the right to invade his neighbour in Moscow's biggest confrontation with the West since the Cold War.
US Secretary of State John Kerry condemned Russia for what he called an "incredible act of aggression".
Russian Foreign Minister Sergei Lavrov discussed Ukraine with his Chinese counterpart on Monday and their views coincided on the situation there, Russia's foreign ministry said.
In a statement, the ministry said the two veto-wielding UN Security Council members would stay in close contact on the issue.