The Ukrainian military on Sunday reported "intensive" movement of troops and equipment from Russia into the separatist controlled parts of eastern Ukraine.

"There is intensive deployment of military equipment and personnel of the enemy from the territory of the Russian Federation onto territory temporarily controlled by insurgents," Ukrainian military spokesman Andriy Lysenko told a briefing in Kiev.

Two pro-Russian separatists are set to be confirmed as leaders in controversial elections in eastern Ukraine on Sunday -- one a rebel chief who highlights his status as a miner's son and the other a fan of Communist icon Vladimir Lenin.

Alexander Zakharchenko, who commands rebels fighting Ukrainian government forces in the mining and industrial town of Donetsk, faces no real opposition in the vote that will make him the first president of the unrecognized Donetsk People's Republic.

The leadership of Ukraine's pro-Russian rebels is on course to secure a crushing victory at Sunday's controversial election that Kiev branded a "farce" and which threatened to deepen an international crisis over the conflict.

Alexander Zakharchenko -- prime minister of the self-proclaimed Donetsk People's Republic -- was estimated to get 81 percent of the presidential vote, according to a rebel exit poll released after the ballot ended. 

Pro-Russian separatists in eastern Ukraine hold controversial leadership elections on Sunday that Kiev and the West have refused to recognize and which threaten to deepen the international crisis over the conflict. 

Fighting raged across the region on the eve of the vote, with seven Ukrainian fighters killed and intensive shelling at the ruins of Donetsk airport, a key battleground between the rebels and government forces.

Six Ukrainian soldiers have been killed by pro-Russian rebels in the last 24 hours, a government official said Saturday.

"Our losses as a result of fighting were six servicemen killed and 10 wounded," Volodymyr Polyovy, spokesman for the National Security and Defense Council, said.

Outside the iconic football stadium in Donetsk in eastern Ukraine, Yulia holds her four-year-old son Maxim by the hand as they wait for the food parcels to arrive.

"I've never come to get humanitarian aid before. I was living off my stored supplies, but I don't have anything left," says Yulia, 30, who lost her job when her company closed down at the start of the conflict between pro-Russian rebels and Ukrainian troops.

Dutch forensics experts have returned to the site of downed flight MH17 in eastern Ukraine on Friday, where they gathered body parts despite continued clashes in the area, Prime Minister Mark Rutte said.

"Today the circumstances allowed for a small Dutch team... to travel to the crash site and we grabbed the opportunity with both hands," Rutte told journalists at his weekly press briefing in The Hague.

Vitaliy Feshchenko, one of thousands of Ukrainian volunteers fighting pro-Russian rebels, has this message for government leaders back in the capital Kiev: his battle-hardened men might come for them next.

The bearded fighter's warning illustrates the lack of trust Ukraine's young revolutionaries have in President Petro Poroshenko and other politicians promising to drag their country from a corrupt, post-Soviet past into a European future.

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.

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.