A fragile truce between pro-Russian insurgents and Ukrainian forces appeared to be consolidate Monday as clashes subsided and attention focused on the unresolved status of the separatist east.

Ukraine said it lost two soldiers in sporadic overnight raids by "armed gangs" on small towns surrounding the main rebel stronghold of Donetsk, but that nevertheless the military was preparing to pull back, as agreed under a new ceasefire deal.

President Petro Poroshenko said Sunday that Ukraine must be ready to defend itself should a peace deal with pro-Russian insurgents fail.

"We must be ready to protect our country if the peace plan does not work," Poroshenko said in a nationally televised interview.

Thousands of Russians on Sunday marched through Moscow to protest against the Kremlin's involvement in the Ukraine crisis, in the country's first major anti-war rally since fighting erupted in April.

A huge column of protesters moved through the heart of the capital to condemn Moscow's role in a conflict that has claimed nearly 3,000 lives and pitted Russians against Ukrainians.

Ukraine insisted Sunday it would not pull back its troops from the frontline until all sides cease fire under a peace plan aimed at ending five months of bloodshed in the east.

Kiev and pro-Russian rebels are supposed to pull back their forces and weaponry to create a 30-kilometer (20-mile) wide buffer zone in line with a reinforced truce deal agreed in Minsk on Saturday.

Relatives of German victims of downed Malaysia Airlines flight MH17 plan to sue Ukraine and its president for criminal negligence for not closing the country's airspace, a lawyer said Sunday.

Elmar Giemulla, an attorney and professor of aviation law who is representing three German families, said he would file suit soon before the European Court of Human Rights in Strasbourg. 

Russian President Vladimir Putin and Japanese Prime Minster Shinzo Abe held telephone talks Sunday amid speculation about whether a visit by Putin will go ahead due to frictions over Ukraine.

Local media reports say Putin's visit to Japan planned for this autumn is now uncertain following Tokyo's sanctions on Moscow over the crisis in Ukraine.

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.

NATO's top military commanders are meeting in Lithuania on Saturday to discuss relations with Russia as the warring sides in Ukraine agreed to pull back troops under a new peace plan reached overnight in Belarus.

General Knud Bartels, who chairs NATO's Military Committee, said defense chiefs from the 28 nation alliance are set to review "future relationship with Russia and NATO's military posture".

Russia has delivered a third shipment of aid to rebel-controlled Ukraine, sending trucks carrying water and food across the border, a spokesman for Russia's emergencies ministry said on Saturday.

"The convoy has arrived and is being unloaded" in the rebel-controlled city of Donetsk, spokesman Alexander Drobyshevsky told AFP.