Police arrested eight Spanish men who returned from fighting alongside pro-Russian forces in eastern Ukraine, in what they said was the first operation of its kind in Europe.

Officers detained the suspects in six regions across Spain after they returned from predominantly Russian-speaking eastern Ukraine, the interior ministry said in a statement.

A Ukrainian airforce pilot, who has been on hunger strike in a Russian prison for 77 days, "could die within days," a member of the Kremlin's human rights council said Friday.

Nadia Savchenko, a 33-year-old helicopter navigator, who has been charged with involvement in the deaths of two Russian reporters in a mortar attack in east Ukraine, has been held in a Moscow jail for nearly nine months.

Russia has confirmed that it will attend talks with the European Union and Ukraine on Monday aimed at resolving a bitter gas supply dispute which threatens deliveries to Europe.

Russian state-owned gas giant Gazprom threatened this week to cut deliveries to Ukraine over a dispute related to Moscow's move to start direct supplies to eastern parts of the country controlled by pro-Kremlin rebels.

Russia's airforce started exercises that will involve jets striking back against a "supposed enemy" over the Barents Sea that borders Norway, the military said Thursday.

Pilots will "practice repelling a massive air strike by a supposed enemy during exercises in the air space of the Barents Sea," the defense ministry said on its website.

The EU said it has invited the Russian and Ukrainian energy ministers to Brussels on Monday for talks on resolving a bitter gas supply dispute which threatens deliveries to Europe.

Russia's state-owned gas giant Gazprom threatened on Tuesday to cut off deliveries to Ukraine, diverting deliveries instead to eastern parts of the country controlled by pro-Kremlin rebels.

Pro-Russian forces in eastern Ukraine will likely wait until the spring to attack the port of Mariupol, US intelligence chief James Clapper predicted Thursday, adding such a move did not seem imminent.

Clapper said Russian President Vladimir Putin's aim was to establish a land bridge to Crimea, which was annexed by Moscow a year ago after the fall of a pro-Moscow leader in Kiev.

The frontline slicing across east Ukraine is more than a boundary fought over by the army and pro-Russian forces; it is also a partition many Ukrainian civilians in the separatist-held zone need to cross to live their lives.

And so, naturally, an underground business has sprung up in providing Ukrainian government passes -- one in which corruption is the byword.

Cyprus played down on Thursday an agreement granting Russian warships access to its ports and denied there was any request from Moscow to open a base on the island.

The heavily indebted eurozone member has close investment and tourism ties with Russia and has repeatedly expressed misgivings about EU sanctions against Moscow.

The European Union called on Russia Thursday to "urgently" release Ukraine military pilot Nadia Savenchko on humanitarian grounds after 75 days on hunger strike to protest her detention.

A spokesperson for EU foreign affairs head Federica Mogherini said Russia "bears a responsibility for her very fragile health" after a Moscow court rejected Savchenko's appeal against her pre-trial detention.

Many NATO member countries are spending less on defense despite promises to boost their budgets in the face of growing tensions with Russia, a think-tank said Thursday.

The European Leadership Network looked at figures for 2015 from 14 NATO members to see if they meet pledges made at a summit of the U.S.-led military alliance last September.