Ukraine said Wednesday it was readying for fresh combat operations in the war-torn east as NATO backed claims that Moscow has poured columns of military hardware across the border.

Kiev's warning will fuel fears of a return to all-out conflict despite a two-month-old ceasefire deal which has halted fighting along much of the frontline but not stopped shelling at strategic flashpoints.

Defence Minister Stepan Poltorak said Kiev was preparing for a new round of fighting after seeing "increased activity" by Russia and pro-Moscow rebels across the region.

"The main task I see is to prepare for combat operations. We are doing this, we are readying our reserves," Poltorak said at the start of a cabinet meeting.

"We observe their movements, we know where they are and we expect unpredictable actions from them."

He said the situation in the conflict zone was "complicated but stable" for now.

His comments came as NATO's supreme commander threw his weight behind allegations that Moscow has stepped up the flow of armaments across the frontier.

"We have seen columns of Russian equipment, primarily Russian tanks, Russian artillery, Russian air defence systems and Russian combat troops entering into Ukraine," US General Philip Breedlove said in Sofia.

Moscow has consistently denied that it is involved in the fighting in east Ukraine.

But it openly gives political backing to the self-declared separatist statelets and it is unclear how else the rebels could have acquired the heavy weaponry typical of a regular army.

Moscow deployed unmarked troops to seize the Ukrainian peninsula of Crimea in March, first denying they were its forces before admitting it had sent in soldiers.

The UN Security Council was set to hold an emergency session to discuss the Ukraine crisis later Wednesday, with representatives from the Organization for Security and Cooperation (OSCE) set to give an update on the situation on the ground.

The OSCE, which is monitoring the ceasefire, warned Tuesday there was a "rising" risk of an escalation in the conflict, which has claimed more than 4,000 lives since April, according to UN figures.

Its observers also reported seeing a convoy of 43 unmarked military trucks -- five towing Howitzer heavy artillery pieces and another five towing multi-launch rocket systems -- travelling into the rebel stronghold of Donetsk on Tuesday.

It was the latest in a string of recent sightings of unmarked trucks and heavy weapons heading towards the frontline in rebel-controlled areas.

- Heavy shelling in Donetsk -

On the ground, several hours of heavy artillery fire rocked Donetsk, the most intense fighting since the weekend.

The explosions of mortars being fired from near the city centre towards government positions at the ruined airport continued throughout the morning but had calmed down by early afternoon, an AFP correspondent said.

Ukraine's military said one soldier had been killed and five wounded as its positions came under repeated shelling around the region over the past 24 hours.

Government forces also accused the insurgents of trying to capture a strategic location along the volatile frontline, delineated as part of the ceasefire deal, north of the second-largest rebel stronghold Lugansk.

The Ukraine crisis has sent relations between Russia and the West plummeting to their lowest point since the Cold War.

Russian President Vladimir Putin was facing renewed Western diplomatic pressure over the conflict during a week of high-level international talks.

Putin discussed Ukraine with US President Barack Obama and Australian Prime Minister Tony Abbott at a trade summit in Beijing on Tuesday.

Australia will host this week's G20 summit in Brisbane which Putin is also to attend amid public anger over allegations Russia supplied rebels with the missile that shot down Malaysia Airlines flight MH17 over eastern Ukraine in July, killing all 298 people onboard including 38 Australian citizens and residents.

Dutch investigators were back at the rebel-controlled crash site on Wednesday looking for body parts but could not say when they would be able to start work on removing the wreckage because of the fragile security situation.