The announcement came as the United States and the European Union stepped up pressure on Moscow, with the White House imposing sanctions on seven Russian officials and 17 firms close to President Vladimir Putin and Brussels adding 15 names to its own list.

Speaking to parliament, Hague criticised Russia as tensions mounted in eastern Ukraine while announcing his plans to visit the area next week.

“I intend to travel to the region next week, including to Ukraine, Georgia and Moldova,” he said, adding that Britain had sent four Typhoon fighter jets to boost NATO’s regular Baltic Air Policing Mission.

“We stand ready to provide more support as NATO’s response develops,” he added.

Georgia and Moldova are the two countries of the former Soviet bloc to have initialled association agreements with the EU.

They made the deal in November when Ukraine pulled out of a similar agreement in favour of closer ties with Russia, sparking the protests that ended the regime of pro-Moscow president Viktor Yanukovych and precipitating Russia’s move to annex Crimea.

Hague revealed he had directly criticised Russian Foreign Minister Sergei Lavrov over his country’s lack of effort to meet the terms of an agreement signed in Geneva last week aimed at easing tensions and staving off the latest round of sanctions.

“Russia has so far failed to implement any part of the Geneva agreement,” said Hague.

“I spoke to Foreign Minister Lavrov last Wednesday. While welcoming his participation in the Geneva meeting I said to him that I could not identify a single positive step Russia has taken to implement the agreement.”

Following the imposition of new sanctions on Monday, Hague said he would not be drawn on what events would lead to further economic punishment for Russia.

“I don’t think it is helpful to try to set out in detail a tripwire or a red line on this… this could invite Russia to go up to that point,” he warned.

“A Russian military invasion of eastern Ukraine would certainly trigger such sanctions… so we stand ready to take such measures.”

The minister “utterly condemned” the capture by pro-Russian militants in Ukraine of international OSCE observers.

“The detention and parading of these observers, who should be released immediately, is utterly reprehensible and does further damage to the standing of Russia and the reputation of such groups operating in Eastern Ukraine,” he said.

“The deplorable shooting today of the Mayor of Kharkiv is another sign of the violence being instigated against those who have opted to support a united Ukraine,” he added.

Britain will on Tuesday host international talks aimed at recovering Ukrainian assets believed to have been looted under the Yanukovych regime.