Russia agreed to resume gas deliveries to Ukraine in an EU-brokered deal reached by the three parties in Brussels on Thursday, an EU source said.

The details will be outlined by European Commission head Jose Manuel Barroso as well as the Russian and Ukrainian energy ministers during a press conference announced for 2045 GMT, the source said.

Russia's most prominent human rights group on Thursday published a list of 46 "political prisoners," including Kremlin critic Alexei Navalny and a Ukrainian pilot detained in Moscow, Nadia Savchenko.

Rights group Memorial publishes an annual list of those it deems to be political prisoners. The Kremlin denies Russia imprisons anyone for political motives but President Vladimir Putin's third term in office has seen a crackdown on the leaders of opposition protests.

NATO chief Jens Stoltenberg said Thursday that the transatlantic alliance remained vigilant after an increase in Russian military maneuvers in European airspace.

"NATO is strong, it remains vigilant," he said during a visit to Athens.

Gazprom on Thursday said that the EU and Ukraine must agree a deal for Kiev to pay off its gas debts to Russia before talks on restarting deliveries to the strife-torn nation can restart.

"The European Commission must reach an agreement with Ukraine over the question of financing. Otherwise, negotiations make no sense," Gazprom spokesman Sergei Kuprianov told AFP.

Iran wants all Western sanctions to be lifted before striking a deal on its contested nuclear program by a November deadline, a top official said Wednesday.

The announcement came amid intensifying efforts to conclude a definitive pact. The six powers in the talks with Iran -- Britain, China, France, Russia, the United States plus Germany, known as the P5+1 -- have set November 24 as the deadline.

NATO said Wednesday it had tracked and intercepted four groups of Russian warplanes "conducting significant military manoeuvres" in European airspace over the past two days.

"These sizable Russian flights represent an unusual level of air activity over European airspace," NATO said.

Ukraine and Russia on Wednesday resumed EU-brokered talks in Brussels aimed at ending a months-long supply cut that also threatens to hit parts of Europe this winter.

European Energy Commissioner Guenther Oettinger told reporters the "common ambition" was to clinch "an interim solution" to assure supplies through the cold season.

Russia announced Tuesday it will recognize separatist polls in Ukraine next weekend, sparking an angry reaction from Washington and Kiev's newly elected pro-Western leaders.

The rebel elections on Sunday should "go ahead as agreed" and Russia will "recognize the results", Foreign Minister Sergei Lavrov told the Izvestia daily.

U.S. Secretary of State John Kerry warned Tuesday that an election planned by separatists in eastern Ukraine would be unlawful, and that Moscow's recognition of the results violates international agreements.

"This will be a clear violation of the commitments made by both Russia and the separatists that it backs in the Minsk agreements," Kerry said during a visit to Ottawa.

Dutch investigators will ask Russia for evidence including of Ukrainian jet fighter activity which Moscow points as the cause of the fatal MH17 air crash, prosecutors said on Tuesday.

"We're in the process of contacting the Russian authorities to see if they have any information that's of importance in the criminal investigation," Wim de Bruin told AFP.