Concerns of a return to full-scale fighting in eastern Ukraine have surged as armoured convoys head to bolster rebel positions in east Ukraine and shelling rocks separatist stronghold Donetsk.

The White House has expressed grave concern at reports of Russian military reinforcements, warning that any separatist efforts to seize more territory would be a “blatant violation” of a ceasefire agreement.

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European Union foreign policy chief Federica Mogherini said reports of military reinforcements in Ukraine’s separatist areas were a “very worrying development” and called on Russia to prevent further movement of “troops, weapons and equipment”.

She also called on all sides to show “restraint” and said Russia should work to find a peaceful solution while respecting Ukraine’s sovereignty.

The early hours of Sunday morning in Donetsk featured some of the fiercest combat since the September 5 signing of a frequently violated ceasefire that halted all-out confrontations across most of the conflict zone.

The Organisation for Security and Co-operation in Europe has voiced concern after its monitors witnessed unmarked columns of tanks and troop carriers moving through east Ukraine in territory held by pro-Russia separatists.

Ukraine’s military has said a large convoy of tanks and other heavy weapons entered the country from Russia across a section of border that has fallen under the control of rebel fighters.

Russia denies being 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 themselves have access to so much sophisticated and well-maintained weaponry.

The OSCE reports fuelled the fears of a total breakdown in the two-month truce, with the war having already killed about 4000 people, according to UN figures.

Ukraine’s military said on Sunday that three servicemen were killed and 13 injured as shelling hit government positions around the region.

Two Ukrainian policemen and one civilian were also killed in the fighting, the army said.

Rebel leader Alexander Zakharchenko risked heavy fire as he toured the insurgents’ forward positions around the ruins of the Donetsk airport, where Ukrainian troops are battling fiercely to maintain a toe-hold.

“They continue to bombard our airport, nothing is changing,” Zakharchenko said.

The conflict has sent relations between Western backers of Ukraine and Russia to their lowest level in decades.

Russian President Vladimir Putin is gearing up for a fraught week of diplomacy with visits to the APEC summit in Beijing and G20 meeting in Brisbane, where he looks likely to face a hostile reception.