The European Union is considering the speedy dispatch of a civilian expert mission to Ukraine to assist with judicial and policing matters, EU diplomats said on Thursday.

The idea was put to ambassadors from the 28-nation bloc jointly by Britain, Poland and Sweden on Wednesday "and was quite well-received," said a diplomat who asked not to be identified.

President Vladimir Putin on Thursday sent a letter to leaders of 18 European countries, warning them Russia could cut gas supplies to Ukraine, but suggesting that Moscow and Brussels work together to help restore the ex-Soviet country's battered economy.

If Ukraine does not settle its 1.6 billion euro ($2.2 billion) energy bill, natural gas giant Gazprom would be "compelled to switch over to advance payment for gas deliveries, and in the event of further violation of the conditions of payment, will completely or partially cease gas deliveries," Putin wrote in the letter.

Russian lawmakers at the Council of Europe's parliamentary assembly were stripped of their voting rights until the end of 2014 on Thursday over Moscow's annexation of Crimea.

The assembly at the Council, a body that has no legislative powers but promotes cooperation on human rights and democracy between European countries, gathers lawmakers from the parliaments of 47 member states, including 18 Russians and 12 Ukrainians.

The embattled acting president of Ukraine promised on Thursday not to prosecute pro-Russian militants occupying government buildings in the east of the country if they laid down their arms and halted a four-day standoff.

The olive branch came as the clock ticked down to a Friday morning deadline for the separatists to walk out of the state security building in the eastern city of Lugansk and the seat of government in nearby Donetsk or face the possible use of force.

Ukraine on Wednesday detained a young Russian activist on suspicion of organizing sabotage during pro-Kremlin protests in which she claims to have shot and wounded several people.

Ukraine's SBU security service said that 22-year-old Maria Koleda had been arrested in southern Ukraine "while carrying out a mission from the (Russian) secret services to destabilize the situation".

NATO denied Russian claims Wednesday that it was planning to deploy massive numbers of troops near the country's border.

"I totally dismiss claim by dep defense minister Antonov that NATO plans to deploy large military contingents close to Russia's borders," said NATO Deputy Secretary General Alexandre Vershbow on twitter.

Third countries should stay out of the debate over Ukraine's future constitution, Poland's prime minister said Wednesday, amid Russia's insistence on solving the crisis through federalization.

"Ukraine's future constitution should, in our opinion, be adopted by Ukrainians alone. And the shape of Ukraine's constitution should not be a topic of discussion for third countries," Prime Minister Donald Tusk told reporters.

Four-way talks between U.S. and EU diplomats, Russia and Ukraine to defuse Europe's worst security crisis in decades look set to be held Thursday next week in Vienna, an EU diplomat said.

"Intense talks are taking place ahead of the four-way negotiations April 17 in Vienna," said the source, speaking on condition of anonymity.

More than 50 people held "hostage" by pro-Russian militants who seized a security building in eastern Ukraine have been released, the state security service said on Wednesday.

Ukraine's SBU security agency said on Tuesday that separatists who had seized its regional headquarters in Lugansk on Sunday had mined the building and were holding 60 people "hostage."

Canada has ordered a Russian diplomat to leave the country, newspapers reported on Tuesday, but officials won't say why.

Lieutenant-Colonel Yury Bezler has been declared persona non grata and has been given two weeks to depart, after serving less than a year as assistant defense attache at the Russian embassy in Ottawa, said the Ottawa Citizen and others, citing unnamed sources.