SHAKHTARSK, Ukraine (AFP) - New unidentified unarmoured columns rumbled toward the pro-Moscow rebel stronghold of Donetsk in eastern Ukraine on Monday as fears grew of a return to all-out fighting in the war-torn region.

The Netherlands meanwhile held an emotional ceremony for the 298 victims of flight MH17 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.

Some 1,600 relatives and friends of the dead from 18 countries gathered alongside members of the Dutch royal family for the sombre event in Amsterdam where the names and ages of those who perished were read out.

Ukraine and the West blame Russia for supplying the pro-Kremlin separatists with the missile that shot down the plane, while Moscow and the rebels have pointed the finger at Kiev's forces.

On the ground in Ukraine, intense weekend shelling around Donetsk and more armoured columns seen heading to the city ratcheted up concerns the rebels could be gearing up for an offensive after weeks of localised skirmishes.

An AFP journalist saw a total of 28 trucks, six tanks and 14 howitzer cannons driving through rebel territory around Donetsk on Monday.

The White House followed Europe by expressing grave concern at reports of Russian military reinforcements, warning that any separatist efforts to seize more territory would be a "blatant violation" of a September ceasefire agreement that had halted full-scale fighting but failed to stop shelling at key flashpoints.

Ukraine's military on Monday repeated allegations that Moscow is sending columns of heavy armour and men across the frontier, saying there was "no doubt" that the deployments were being commanded by the Russian army.

Russia has repeatedly denied that it is involved in the fighting in the east.

However, it openly gives the rebels political and humanitarian backing and it is not clear how the insurgents could otherwise have access to so much sophisticated and well-maintained weaponry.

AFP journalists reported sporadic shelling in Donetsk overnight, some 24 hours after heavy bombardments rocked the rebel bastion in some of the fiercest fighting there since the September 5 truce deal.

Two Ukrainian soldiers were killed and five wounded as government positions came under artillery fire around the region, the military said. Some 4,000 people have died in the war since April, according to UN figures.

At the MH17 ceremony in Amsterdam, large candles burned in memory of the victims, 193 of whom were Dutch citizens.

"They will not be forgotten, the beautiful warm and touching memories are forever," Dutch Prime Minister Mark Rutte said.

Anton Kotte, who lost three relatives in the crash, said for the bereaved the time since the disaster had been like being stuck together on a train that never stopped.

"We didn't know each other but we are partners in misfortune," said Kotte.

The remains of 289 of the victims have so far been identified but the Dutch foreign minister has admitted that the remaining dead may never be recovered as the security situation around the crash site remains volatile.

Russian President Vladimir Putin said Monday that shelling at the location by Ukrainian forces was preventing an international probe at a meeting with Malaysian Prime Minister Najib Razak on the sidelines of the Asia-Pacific Economic Cooperation summit in Beijing.

Dutch investigators have made four visits to the site since the signing of the ceasefire in September, but The Hague says it does not know when they will be able to recover the wreckage of the plane.

The conflict in Ukraine has sent relations between Western backers of Ukraine and Russia to their lowest levels since the end of the Cold War.

Russian President Vladimir Putin embarked on a fraught week of diplomacy at the Apec summit in Beijing and Group of 20 meeting in Brisbane, Australia, where he looks likely to face a hostile reception from Western leaders.

Russia's economy is suffering from European Union and US sanctions imposed in response to Moscow's support for the separatists, with the ruble losing some 25 percent of its value this year.

But Putin sounded a confident note as he shrugged off concerns over the ruble's collapse, putting it down to short-term speculation as the national currency rallied Monday.

"I think it will stop soon given the actions taken by the central bank in response to speculators," Putin said in a speech in Beijing.