The prospect of a European energy crisis loomed last night as Moscow threatened to impose retaliatory sanctions, and the new Prime Minister of Ukraine said his country could suffer shortages if help was not received from the West.
Russia supplies 30 per cent of Europe's gas and a number of the Baltic states are almost entirely reliant on Russia.
Yesterday, the Russian foreign ministry labelled this week's EU sanctions, in which a further 12 people close to President Vladimir Putin have had their assets frozen and a travel ban imposed, as "divorced from reality".
With both Brussels and Washington now threatening further "targeted measures" this week, Moscow made it clear that it was considering tit-for-tat sanctions. "Russia reserves the right to give an adequate response to the undertaken action," a spokesman said.
Ukraine's Prime Minister, Arseniy Yatsenyuk, highlighted precarious energy supplies during a visit to Berlin, saying gas from the EU might be needed. He has previously said he fears Moscow plans to use energy as a "new nuclear weapon" to intimidate its neighbours.
In 2006, when Russia cut off supplies over a dispute with Ukraine over gas prices, many parts of Europe ran out of gas. Bulgaria shut down its main industrial plants, and Slovakia declared a state of emergency. The situation is now far less worrying. Reserves are high and Europe's gas supply system has improved following a £1.3bn investment. G7 leaders will meet in The Hague tomorrow, and energy security and Ukraine's national elections on 25 May are due to be on the agenda.
In pictures: Ukraine crisis
In pictures: Ukraine crisis
1/12 Ukraine crisis
People shout slogans during a pro Russian rally at a central square in Donetsk. Pro Russian activists continued to gather on Saturday in the eastern Ukrainian city of Donetsk, as Russia was reported to be reinforcing its military presence in Crimea.
2/12 Ukraine crisis
In the same pro Russian rally, demonstrators show their support. Ukraine's ambassador to Russia and a deputy Russian foreign minister held a "cordial" meeting on Saturday, Moscow said, without giving details of any discussion of Russian-occupied Crimea.
3/12 Ukraine crisis
Crimean ethnic tatars stand on the roadside as Russian troops move towards to Simferopol in the settlement of Kok-Asan, some 70 kilometres from Simferopol in Crimea.
4/12 Ukraine crisis
Russian troops stand on a roadside in the settlement of Opytnoye, some 70 kilometres from Simferopol.
5/12 Ukraine crisis
Armed members of the first unit of a pro-Russian armed force, dubbed the "military forces of the autonomous republic of Crimea" march before the swearing-in ceremony in Simferopol, Ukraine. Some 30 men armed with automatic weapons and another 20 or so unarmed, were sworn in at a park in front of an eternal flame to those killed in World War II.
6/12 Ukraine crisis
A group of Cossacks march past a statue of Soviet revolutionary leader Vladimir Lenin in Simferopol as tensions in the area continue to rise.
7/12 Ukraine crisis
An armed member of the first unit of a pro-Russian armed force, dubbed the "military forces of the autonomous republic of Crimea" signs the oath during the swearing-in ceremony in Simferopol,
8/12 Ukraine crisis
9/12 Ukraine crisis
Ukrainian soldiers load their armed personnel carriers (APCs) into boxcars in the western Ukrainian city of Lviv. Pro-Kremlin militia fired warning shots as unarmed foreign observers tried to enter Crimea on the 8th.
10/12 Ukraine crisis
An abandoned naval ship sunk by the Russian navy to block the entrance is seen in the Crimean port of Yevpatorya on March 8th.
11/12 Ukraine crisis
Ukrainian sailors stand guard on top of the Ukrainian navy ship at the Crimean port of Yevpatorya.
12/12 Ukraine crisis
Crimea's pro-Moscow leader Sergei Aksyonov speaks to the media in Simferopol on the 8th March. He has defended a decision to hold a referendum on whether the region should join Russia, saying on Saturday that "no one" could cancel the voting.
Yesterday, as Russian troops stormed the last remaining Ukrainian-held airbase in Crimea, more than 5,000 people gathered in the eastern city of Donetsk to demand a referendum on whether they, too, could split and join Russia.
Last night, the Foreign Office issued new advice warning against "all but essential" travel to Donetsk, as well as Lugansk and Kharkiv due to "regional tensions".