TYCHY, Poland (REUTERS) - Polish Prime Minister Donald Tusk said on Saturday the European Union should form an energy union to bolster its energy security and lessen its dependence on key gas supplier Russia whose annexation of Crimea has caused a tense stand-off with the West.

Russia, which provides around one third of the EU's oil and gas, sent shockwaves through the international community with its military intervention and annexation of Ukraine's Crimea peninsula two weeks ago.

The action prompted the United States and its European allies to begin imposing sanctions on President Vladimir Putin's inner circle and to threaten to penalise key sectors of Russia's economy.

Some 40 per cent of Russia's gas destined for Europe is shipped through Ukraine.

"The experience of the last few weeks shows that Europe must strive towards solidarity when it comes to energy," Tusk said.

He said Poland's proposed energy union would be based on six points including the "rehabilitation" of coal as a valid energy source, more shale gas exploration and common purchases to ensure the best price.

Liquid gas imports from the United States should also play a role in Europe's energy diversification efforts, he said.

Since Russia's annexation of Crimea, European leaders have already agreed to accelerate their quest for more secure energy supplies and reduce dependence on Russian oil and gas.

The EU has made progress in improving its energy security since gas crises in 2006 and 2009, when rows over unpaid bills between Kiev and Moscow led to the disruption of gas exports to western Europe.

However, it has not yet managed to reduce Russia's share of European energy supplies.