Spanish police on Wednesday detained Ukraine's former finance minister Yuri Kolobov who is wanted by Kiev for alleged fraud and theft of public funds, a police source said.

The 41-year-old, who served under ousted pro-Kremlin president Viktor Yanukovych, was arrested in Altea, a hilltop village on the Mediterranean coast, the source said.

The United States' senior envoy to Europe alleged Wednesday that Russia had deployed "thousands and thousands" of troops to neighboring Ukraine.

Speaking to a congressional foreign affairs committee, Assistant Secretary of State Victoria Nuland also told U.S. lawmakers that Russia was flooding eastern Ukraine with military hardware.

German Chancellor Angela Merkel warned Russia Wednesday of new sanctions if the Ukraine ceasefire accord is not fully implemented, insisting that Kiev be allowed to regain full control of its eastern border.

"One thing is absolutely clear, if the Minsk ceasefire accord does not work then (EU) member states and the European Commission are quite prepared to move to new sanctions," Merkel said.

Ukraine's President Petro Poroshenko said Wednesday he has sent a letter to Russia's Vladimir Putin asking him to free jailed pilot Nadiya Savchenko, who has been on a hunger strike for nearly three months.

"I have sent a letter to President Putin with an appeal for immediate freedom for Nadiya Savchenko, including for medical reasons," Poroshenko said in a press conference after meeting Italian premier Matteo Renzi.

The EU launched an in-depth review of policy towards its regional neighbors Wednesday in response to the Ukraine crisis and turmoil across North Africa and the Middle East.

In 2003 the bloc unveiled its first European Neighborhood Policy confident it could establish close links with former Communist states in eastern Europe, including Russia, and with then-stable governments such as that of president Hosni Mubarak in Egypt.

Kremlin opponent Boris Nemtsov's blood was barely dry on the Moscow sidewalk before powerful Russians came up with a startlingly clear conclusion: that the West was to blame.

The West stands accused of an increasing number of outrages in President Vladimir Putin's Russia.

At least 10 miners were killed and 23 more were missing and feared dead in eastern Ukraine after a blast on Wednesday at an accident-prone coal mine in the rebel-held city of Donetsk.

The early morning blast, believed to be caused by a build-up of methane gas, hit the Zasyadko mine near the city's bomb-scarred airport in what officials described as a "terrible tragedy".

Western leaders on Tuesday called for a "strong reaction" from the international community if a major violation of a truce in Ukraine occurs, the French presidency said after video conference talks.

The leaders of the United States, France, Germany, Britain and Italy as well as EU head Donald Tusk held video talks on the Ukraine conflict on Tuesday.

The first of around 500 relatives of those killed in the shooting down of Malaysia Airlines flight MH17 over Ukraine visited the wreckage at a Dutch airbase on Tuesday, investigators said.

The wreckage was brought to the Gilze-Rijen base in the northern Netherlands late last year as part of a probe into what exactly shot down the Boeing 777 in July, killing all 298 people on board.

A Ukrainian airforce pilot who has been on hunger strike in a Russian jail for 81 days will be transferred to a civilian hospital if her health deteriorates, the prison service said Tuesday.

The statement by Russia's prison service raised the possibility of Nadia Savchenko, who is also a member of the Ukraine parliament, being transferred from the hospital of a Moscow prison where she has been held for nearly nine months.