The European Union said Friday that Russian President Vladimir Putin had written to Brussels demanding changes to a landmark EU-Ukraine accord, but it ruled out reopening the deal without Kiev's consent.

Putin's letter to European Commission chief Jose Manuel Barroso reportedly threatened retaliatory measures if the EU and Ukraine do not stick to their agreement with Moscow to delay the implementation of the deal until 2016.

Pro-Russian separatists in eastern Ukraine and authorities in Kiev are planning to hold talks on energy in the region as winter looms over the strife-torn territory, a separatist leader told Agence France-Presse Thursday.

"These discussions will be about vital infrastructure, including about coal and electricity," Andrei Purgin, deputy prime minister of the self-proclaimed Donetsk People's Republic, told AFP by phone. "They need our coal."

Russia is acting like a "terrorist" in Ukraine and in its intimidation of other neighbors, Lithuania's President Dalia Grybauskaite said in an interview released Thursday.

"I think that Russia is terrorizing its neighbors and using terrorist methods," she told the Washington Post newspaper, replying "Yes" when asked whether she thought both Russia and the Islamic State (IS) group, which is sweeping across Syria and Iraq, were terrorists.

Leaders of the world's most developed economies Thursday warned Russia of more sanctions unless all fighting stops in Ukraine and vowed to stand by the war-torn country as winter approaches.

"Sanctions are not an end in themselves; they can only be rolled back when Russia meets its commitments related to the ceasefire and the Minsk agreements and respects Ukraine’s sovereignty," G7 leaders said after talks in New York.

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.

U.S. President Barack Obama denounced Russian aggression in Europe at the United Nations on Wednesday, but proffered a hand to Moscow by offering to lift sanctions if Russia changes course.

"Russian aggression in Europe recalls the days when large nations trampled small ones in pursuit of territorial ambition," Obama told the General Assembly with Russian Foreign Minister Sergei Lavrov in the audience.

Polish anti-communist icon Lech Walesa said Poland should procure nuclear weapons as a safeguard against Russia, which it blames for stoking the crisis in neighboring Ukraine.

"Poland needs to stand up to Russia," the Nobel Peace laureate, who spearheaded Poland's democracy movement and became its first post-communist president, said in an interview published Wednesday.

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.