Ukrainian President Petro Poroshenko said on Thursday he was confident that his peace deal with pro-Russian insurgents means the "most dangerous" part of the five-month conflict is over.

"I have no doubt that my peace plan will work. I have no doubt that the biggest, most dangerous part of the war is already behind us, thanks to the heroism of Ukrainian soldiers," Poroshenko said at his first press conference since he assumed office in early June.

President Petro Poroshenko on Thursday ordered a temporary closure of Ukraine's porous border with Russia and voiced plans to apply for EU membership in 2020 as part of his ex-Soviet country's Westward shift.

A senior Ukrainian security source told AFP that the border security measures was designed to halt the alleged smuggling of weapons into the separatist east and would enter into force "soon".

U.S. President Barack Obama on Wednesday slammed Russia's "aggression" in Ukraine but offered to lift sanctions against Moscow if it threw its weight behind an unraveling peace deal with Kiev.

The outstretched hand to Moscow came a day after pro-Russian guerrillas brushed off Ukrainian President Petro Poroshenko's limited self-rule offer and announced plans to set up their own parliaments in self-organized November 2 polls.

Japan hit Russia with further sanctions on Wednesday over the crisis in Ukraine, in a further blow to efforts to end a longstanding territorial row between the pair.

The fresh penalties, targeting exports of military equipment and technology to Russia, come a day after Japanese Prime Minister Shinzo Abe reportedly dropped plans to host President Vladimir Putin later this year due to tensions over the conflict.

Ukrainian peace efforts stalled Wednesday after pro-Russian insurgents called their own elections in defiance of a deal under which they and the Ukrainian army began withdrawing heavy weapons after five months of war.

Separatists in the Russian-speaking industrial east Tuesday brushed off President Petro Poroshenko's limited self-rule offer and announced plans to set up independent parliaments in self-organized polls on November 2.

The death toll in Ukraine's conflict has soared past 3,000, likely by a significant margin, top U.N. human rights official Ivan Simonovic said Tuesday.

"The current registered death toll, as at 21 September, is 3,543, if we are to include the 298 victims of the Malaysian plane crash," Simonovic told the U.N. Human Rights Council.

Pro-Russian rebels defiantly announced Tuesday they would stage their own elections in November, raising the stakes in a standoff with Kiev despite both sides moving to end five months of deadly fighting.

The prime minister of the self-proclaimed Donetsk People's Republic, Alexander Zakharchenko, said the eastern separatist region would hold elections to choose a parliament and a leader on November 2.

Poland kicks off major military exercises involving eight NATO partners on Wednesday amid the West's worst standoff with Russia since the end of the Cold War sparked by differences over the Ukraine crisis.

Organized every two years since 2006, this year's Anaconda maneuvers involve 12,500 soldiers, with 750 from NATO members the U.S. and Britain among others, the Polish defense ministry said, adding the event "has become a permanent training element of the North Atlantic alliance".

The prime minister of the self-proclaimed People's Republic of Donetsk said Tuesday that the pro-Russian rebels have removed artillery from frontline areas where Ukraine had also withdrawn, in line with a peace plan signed Saturday.

"We have withdrawn artillery but only in those areas where the Ukrainian regular units have done the same. Where Ukraine hasn't withdrawn artillery, we also haven't done so," the rebel leader, Alexander Zakharchenko told the Interfax news agency. 

A senior separatist from eastern Ukraine said on Monday he believed there was a chance a peace plan for eastern Ukraine could succeed.

"The agreements will be carried out, but with great difficulty," Andrei Purgin, the deputy prime minister of the self-proclaimed Donetsk People's Republic, told Agence France-Presse.