The US defence department has claimed that Russia is massing more troops near Ukraine's border, and warned that the movement of heavy calibre artillery systems across the border could be "imminent".

Colonel Steve Warren, a Pentagon spokesman, said on Friday that the US had detected the systems moving closer to the border and they could be put into the hands of Russian-backed separatists immediately.

"This is unquestionably an escalation from a military perspective," he said, adding that he does not have exact timeline.

Warren also said that the number of Russian troops along the border also continued to slowly increase from the current 12,000. The US ambassador to NATO, Douglas Lute, estimated the number was now 15,000.  

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Warren also said Russia continued to fire artillery shells across the border into Ukraine, backing earlier allegations by the Ukrainian government.

However, Al Jazeera's Rory Challands, reporting from Moscow, said the US was having "a bit of a credibility issue" - providing no proof of its claims, which were therefore easy to dismiss.

Countering the US and Ukrainian allegations, Russia accused Ukraine of firing into Russian territory.

"Around 40 mortar shells have fallen from Ukraine ... in Rostov province," Vasily Malayev, a border guard spokesman, told the state Ria Novosti news agency.

Overnight, the Ukrainian army said that its soldiers had come under artillery fire from the Russian side of the border. The government's military listed at least seven locations where rebels attacked Ukrainian troops.

Moscow has denied a role in the conflict between pro-Russian separatists and government troops which has left more than 400 people dead and displaced tens of thousands.

On Thursday, the US said it has evidence that Russia is firing artillery from within Russia to attack Ukraine military positions.

Meanwhile, a truce has been declared in the vicinity of the vast Malaysian Airlines crash site in rebel-held Grabove, where experts say some remains of the victims still lay decomposing under the sweltering summer heat more than a week after the tragedy.

Dutch authorities said 189 coffins have been flown to the Netherlands where the remains would be identified, with another flight set to carry 38 more from the eastern Ukrainian city of Kharkiv to Eindhoven on Saturday.

In capital Kiev, president Petro Poroshenko sought to avoid political crisis and called on parliament to heed "cold reason" and pass a vote of confidence in the government, a day after premier Arseniy Yatsenyuk stepped down.

Yatsenyuk's resignation piles on more woes for a country already struggling to cope with a chaotic situation in the rebel-controlled east, where international experts are carrying out a complex investigation into last week's downing of MH17 that killed 298 people.

Source: Al Jazeera and agencies