Russia has begun a build-up of armoured vehicles on its side of a narrow stretch of water between Russia and the Ukrainian region of Crimea, Ukrainian border guards said.

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

He said the build-up of Russian armour was near a ferry port on what is known as the Kerch Strait.

According to reports, pro-Russian troops have taken over the ferry terminal on the easternmost tip of Crimea, exacerbating fears that Russia is planning on moving more troops into the Black Sea region.

"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.

The Ukrainian defence ministry also reported that Russian fighter jets twice violated its airspace during the night, forcing it to scramble an interceptor aircraft to prevent and "provocative actions".

There was no immediate comment from the Russian defence ministry.

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.

The prime minister of Ukraine's new government, Arseniy Yatsenyuk, says Russia has invaded his country.

"President [Vladimir] Putin wants to be the president who started the war between two neighbouring and friendly countries - between Ukraine and Russia," he said.

"So he has reached this target with a few inches. We are on the brink of the disaster."

Mr Yatsenyuk's pro-Western government took power when former president Viktor Yanukovych, a Russian ally, fled on February 21 after three months of street protests against his rule.

World powers condemn Russia

The world's top economic powers have condemned Russia for its military actions in Crimea and its build-up on the Ukraine border.

G8 members Britain, US, Canada, France, Italy, Germany, and Japan said Russia's actions are a "clear violation" of Ukraine's sovereignty.

Sorry, this video has expired Ukraine prepares for war with Russia ( Matt Brown )

Expert analysis: Journalist Charles McPhedran talks to PM's Mark Colvin about the reaction in Europe to Russia's military moves. It's very hard to make generalisations across the east. Essentially in Ukraine, you have provinces surrounding big cities and each of the dynamics in those big cities in the south and the east is different. I think that Europe is doing everything it can to confine the conflict with Crimea... and if those pro-European parts of western Ukraine get pulled into this conflict, that could lead to a lot of bloodshed. I think even losing Crimea would be a bitter pill to swallow for people. Having fought for so many weeks and so many months on Independence Square, in some cases literally fighting, I think in some cases it's almost unthinkable for people in the west and in Kiev to give up to Russian forces. I think that the reaction from eastern European nations would be unpredictable as well.

Symbolically billing themselves as the G7, they warned in a statement that Russia's actions were incompatible with the G8 nations, which Moscow joined in 1997.

They also said they would not take part in preparatory talks for June's G8 summit in Sochi, Russia.

NATO and the United Nations are also increasing the pressure on Russia as tensions rise over its actions on the Crimean Peninsula.

"I appeal to president Putin to urgently engage in direct dialogue with the authorities in Kiev," UN secretary general Ban Ki-moon said.

Britain says it is very concerned about the possibility that the Kremlin might send troops further into Ukraine and cautioned Mr Putin that Russia would pay significant costs unless he changed course.

British foreign secretary William Hague said the Russian intervention in Ukraine was the biggest crisis in Europe this century.

He said that while Russia had the legal right to base troops in Crimea, the Kremlin should order them to return to their barracks.

"This is a very tense situation and dangerous situation that Russia's intervention has now produced," he told the BBC.

He also warned of "consequences and costs" for Russia.

However, after holding talks with Ukrainian leaders in Kiev, he specifically ruled out military intervention and stressed the British were seeking diplomatic and economic measures.

He also pledged 10 million pounds of technical help for economic and political reform in the country.

Meanwhile, China and Russia are in broad agreement on the situation in Ukraine, the Russian foreign ministry said after telephone talks between the countries' top diplomats.

Russian foreign minister Sergei Lavrov and his Chinese counterpart Wang Yi noted "broadly coinciding points of view of Russia and China over the situation that has developed in the country and around it"m the Russian foreign ministry said in a statement.

ABC/Reuters