Seven Ukrainian soldiers have been killed in the past day of fighting with pro-Russian rebels in the east of the country, the military said Thursday, the biggest daily toll in more than two weeks.

"Over the last 24 hours, we have lost seven servicemen, 11 were injured," Ukrainian military spokesman Andriy Lysenko told reporters.

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.

As winter approaches in Ukraine's eastern countryside, pro-Russian rebels and Kiev forces eye each other uneasily across a ragged frontline. Despite signing a truce, both seem to be digging in for the long haul.

"We are repairing this tank after it was hit by mortar shells yesterday. The war will be long. If Kiev wanted peace it would have finished a long time ago," a rebel commander of a checkpoint near the village of Lukove said.

U.N. Secretary General Ban Ki-moon on Wednesday deplored plans by Ukrainian separatist rebels to hold elections at the weekend, saying the polls undermined peace accords.

The vote on Sunday in east Ukraine "will seriously undermine the Minsk Protocol and Memorandum, which need to be urgently implemented in full," Ban said in a statement released by his spokesman.

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.

NATO wants a constructive relationship with Russia but for that to happen it must engage Moscow from a position of strength, alliance head Jens Stoltenberg said Tuesday.

The US-led military pact had helped ensure stability in Europe, he said, but now, with its intervention in Ukraine, Russia was "trying to roll back the progress we have made".

Russia announced Tuesday it will recognize separatist polls in Ukraine next weekend, fueling tensions with the country's newly elected pro-Western leaders as they negotiate on forming a coalition government.

The rebel elections on Sunday should "go ahead as agreed," Russian Foreign Minister Sergei Lavrov said.