U.S. Vice President Joe Biden warned Russia over Ukraine after new columns of tanks, trucks and heavy artillery rumbled towards the pro-Moscow rebel stronghold of Donetsk on Monday.

The West has repeatedly expressed concern at Ukrainian claims that Russian military reinforcements are being sent in, while Moscow denies that it is involved in the fighting in the eastern part of its neighbor.

The Netherlands on Monday held a sombre and emotional memorial for the 298 people who died when Malaysia Airlines flight MH17 was shot down over war-torn eastern Ukraine in July.

"What if the holiday had started a day later? What if the plane had been late? What if I wake up and realize it was all just a dream?" Prime Minister Mark Rutte told the memorial ceremony in Amsterdam, attended by 1,600 of the bereaved.

Russian President Vladimir Putin on Monday said that the investigation of the MH17 crash site is stalling due to Kiev's constant shelling of the rebel-controlled area in eastern Ukraine.

Putin told Malaysian Prime Minister Najib Razak that rebels were not hindering access to the site of the Malaysia Airlines crash, in which 298 people died in July when the aircraft came down in an area controlled by pro-Russian separatists.

New unidentified armored columns rumbled toward the pro-Moscow rebel stronghold in eastern Ukraine on Monday as fears grew of a return to all-out fighting in the war-torn region.

The Netherlands meanwhile was set to hold an emotional ceremony for the victims of the MH17 flight shot down over the separatist-held eastern Ukraine in July in a tragedy that brought the conflict in the ex-Soviet state to the international fore.

The leaders of China and Japan held an ice-breaking summit Monday after two years of dangerous animosity, as world leaders including Barack Obama gathered for an Asia-Pacific meeting spotlighting intensifying big-power rivalries.

Xi Jinping and Shinzo Abe met in Beijing's Great Hall of the People in what the Japanese prime minister said was a "first step" towards repairing the fractured relationship between the world's second- and third-largest economies.

Armored convoys headed to bolster rebel positions in east Ukraine Sunday as shelling rocked separatist stronghold Donetsk and fears mounted of a return to full-scale fighting.

Shelling rumbled on throughout the afternoon on the edge of Donetsk, where government forces regularly exchange heavy fire with insurgent fighters, but was less intense than overnight when mortar fire was heard close to the center for around two hours, an AFP journalist reported.

The Soviet Union's last leader Mikhail Gorbachev warned Saturday that the world was on the "brink of a new Cold War" at an event to mark 25 years since the Berlin Wall's fall, the German news agency DPA reported.

"The world is on the brink of a new Cold War. Some are even saying that it has already begun," the 83-year-old said, amid tensions between the West and Russia over Ukraine.

Two Ukrainian doctors kidnapped in Libya have been freed," Ukrainian Foreign Minister Pavlo Klimkin said Saturday.

"After huge efforts, the two Ukrainian doctors kidnapped in September in Libya have been freed. They are in a safe place," Klimkin said on his Twitter account.

The last remains of the victims from downed Malaysia Airlines flight MH17 might never be fully recovered, the Dutch foreign minister said Saturday, as five more coffins were flown out of Ukraine. 

"We cannot say at this moment in any certain way at what moment and even if we can recover the last nine (victims), but we will do everything we can in cooperation with authorities here to make that happen and that work is still possible to do," Foreign Minister Bert Koenders told journalists after a ceremony at Kharkiv airport.

Russia's top diplomat Sergei Lavrov said Saturday that U.S. involvement in resolving the conflict in Ukraine would be a "step in the right direction", after meeting with his U.S. counterpart John Kerry in Beijing.

Lavrov's remarks appeared to soften Russia's line on the crisis ahead of high-level meetings between President Vladimir Putin and world leaders at an APEC summit in China and a Group of 20 summit in Australia next week.