Pro-Russian activists occupy the regional security service building in Lugansk, eastern UkraineAFP

Ukraine has not settled its $2.2bn unpaid Russian gas bill, despite a Gazprom-imposed deadline passing at midnight on Monday.

As the deadline passed, a Gazprom spokesman told Reuters that "there have been zero payments from Ukraine".

It is unclear what the consequences of the non-payment might be; Ukraine has missed some deadlines in the past without punishment.

However disputes between the two parties over gas prices in 2005 and 2008 led to Gazprom cutting off Ukraine's gas supply.

Of course worsening political relations between Kiev and Moscow means concessions are unlikely.

The gas stand-off began last week after Gazprom hiked the price it charges Ukraine by almost double. By the weekend, it was charging Ukraine $485 per 10,000 cubic metres of gas.

Ukraine's interim Prime Minister Arseniy Yatseniuk called the rises a form of economic aggression and that they were politicised.

Meanwhile, Ukraine's energy minister is due to meet with European Union officials on Tuesday to discuss a possible deal over the country obtaining gas from the European bloc.

Ukraine currently relies on Russia for around half of its gas, while the EU imports around a third of its gas from Russia, 40% of which flows through pipelines in Ukraine.

EU officials told Reuters that Ukraine's Energy Minister Yuri Prodan will meet EU Energy Commissioner Guenther Oettinger on Tuesday. They're likely to discuss a proposal that would see Ukraine importing gas from Slovakia.

However, that plan was cast in to doubt on Monday when a spokesman for Slovakia's pipeline Eustream said that the move to reverse the flow of gas could need permission from Gazprom.

"For a reverse flow, you would have to stop the East-West flow in one of the pipelines and reverse the flow. But you would have to have approval from Gazprom," the spokesman said

"Gazprom does not agree with this, and thus it's not an option," he added.

Moreover, tensions between the neighbours were stretched further after a number of pro-Russian protestors have stormed government buildings and seized arms in one city, while declaring a separatist republic in another. US Secretary of State John Kerry voiced "great concern" over the heightened tension.