The United States said Sunday it hoped Russia would address concerns about its actions in Ukraine ahead of the G20 November leaders' summit, or Moscow risked further economic isolation.

U.S. Treasury Secretary Jack Lew said following a two-day meeting of G20 finance chiefs "the goal between now and November is for this to be resolved so that these issues are on a path towards improvement.

Mikhail Khodorkovsky, a Kremlin opponent who spent a decade in prison, on Saturday launched a movement to bring together pro-European Russians to challenge strongman Vladimir Putin's grip on power.

Once Russia's richest man, Khodorkovsky was released from prison late last year and is now based in Switzerland. He urged his compatriots to pull together to be able to influence the fate of the country.

Vladimir Putin is set to attend the G20 leaders' summit in November despite alarm over Russia's actions in Ukraine with host Australia Saturday saying it was the "emphatic" wish of member nations.

There had been question marks over whether Moscow should be invited to the high-powered forum in Brisbane with the brutal five-month conflict that has killed around 3,000 people still unresolved.

Kiev accused Moscow on Thursday of massing its troops in annexed Crimea on the Ukraine border, rattling nerves just as President Petro Poroshenko prepared to meet U.S. counterpart Barack Obama.

The apparent push north by about 4,000 troops in the Black Sea peninsula came despite Russia's declared backing of a peace overture by Kiev to try to end five months of conflict in the rebellious east.

Neither Iran's Hassan Rouhani nor Russia's Vladimir Putin feature on President Barack Obama's "dance card" of meetings with foreign leaders at the U.N. next week, the White House said Monday.

Speculation about possible talks between Obama and Rouhani in particular has been mounting because the counterparts spoke by phone last year on the sidelines of the annual United Nations General Assembly.

The Ukrainian military said Saturday its forces had repelled an insurgent attack on the airport in the eastern separatist stronghold of Donetsk, despite an eight-day ceasefire.

"Many rebels backed by six tanks launched an assault against the airport on Friday which was heroically repelled by the soldiers," the press service for the military operation in eastern Ukraine said.

Russian President Vladimir Putin on Wednesday emerged from a Moscow church and told journalists he had lit a candle in memory of those who died defending separatist eastern Ukraine.

"Who did I light a candle for? For those who were injured and those who gave their lives defending people in Novorossiya," Putin said, shown on Russian television.

President Vladimir Putin on Wednesday accused Western countries of creating the Ukrainian crisis and using it to revive NATO.

Speaking after the Western military bloc agreed to boost its presence in eastern Europe, the Russian strongman warned Brussels and Washington against "any hysterics" if Russia chooses to retaliate.

Ukrainian President Petro Poroshenko visited a flashpoint eastern city Monday where sporadic fighting has rattled a three-day-old ceasefire between government and rebel forces.

The highly-symbolic trip to the strategic port of Mariupol came just as EU officials called a new meeting to decide on fresh sanctions against Moscow should the truce fail.

Fighting around two flashpoint cities in eastern Ukraine on Sunday rattled a tenuous truce between government troops and pro-Russian rebels less than 48 hours after it came into force.

Insurgent militias bombarded a government-held checkpoint on the eastern edge of the port city of Mariupol overnight, local officials said, killing one woman and triggering panic among residents.