Kiev, the United States and Nato accused Russia of continuing to send troops and heavy weaponry to support rebels in eastern Ukraine, as top diplomats prepared for talks aimed at ending the latest surge of deadly fighting in the region.
Foreign ministers from Ukraine, Russia, Germany and France were due to hold discussions on Wednesday night at a villa near Berlin airport, but German chancellor Angela Merkel discouraged hopes of a major breakthrough.
"We have more than 9,000 troops of the Russian Federation on my territory, including more than 500 tanks and heavy artillery and armed personnel carriers," Ukrainian president Petro Poroshenko said in Davos, Switzerland.
“If this is not aggression, what is?” he asked an audience of influential political and financial figures, to whom he showed a shrapnel-scarred piece of a bus that was allegedly hit by rebel shelling this month, killing 13 passengers.
The audience applauded when Mr Poroshenko described what Russia should do to end a conflict that has killed more than 4,800 people and displaced about one million: “Stop supplying weapons. Stop supplying ammunition. Withdraw the troops and close the border. A very simple peace plan.”
Mr Poroshenko said he would return home early from the World Economic Forum due to a sharp increase in fighting in recent days, which Kiev blames on a new influx of Russian troops crossing into rebel-held parts of the Donetsk and Luhansk regions.
Clashes continued around Donetsk airport on Wednesday, and heavy exchanges of artillery fire were reported at several other points close to the line demarcating government- and rebel-held territory according to a September “ceasefire” deal.
The combatants accuse each other of breaking the pact, and of firing shells into residential areas which kill and injure civilians almost every day.
In Brussels, Nato secretary general Jens Stoltenberg said Russia's military was moving "back and forth over the border" and now had "a high number of forces on the border".
Nato was now seeing “an increase in Russian equipment . . . tanks, artillery, armoured vehicles and advanced air defence systems,” Mr Stoltenberg said.
In Kiev, the visiting commander of the US army in Europe, Ben Hodges, said it was “irrefutable that [the insurgents] are getting direct support from Russia”.
"It is very clear from the capabilities that the proxies [rebels] have used against Ukrainian security forces, the type of artillery, modern equipment, the amount of ammunition," Lt Gen Hodges said, adding that some of the high-tech weaponry could only be operated by trained specialists, not a rag-tag militia.
Moscow denies such accusations, however, insisting that any Russians fighting in Ukraine are volunteers, and that all rebel weapons were seized from government forces and stocks.
"If you allege this so confidently, present the facts. But nobody can present the facts, or doesn't want to," Russian foreign minister Sergei Lavrov said in Moscow before flying to Berlin.
He insisted the militants would withdraw heavy weapons to areas stated in the September deal agreed in Minsk, but did not commit Russia to closing its border with rebel-held areas – a condition that Ukraine and the West say is crucial.
“Russia will do all it can to achieve an immediate ceasefire,” he said.
“Western countries should urge the Ukrainian leadership not to allow recourse to military conflict. I hope common sense will prevail.”
Dr Merkel played down suggestions that Wednesday night’s four-way talks would pave the way for a summit of the countries’ leaders.
“We don’t want another meeting of presidents that yields no results,” she said.
“I don’t want to get hopes up too much . . . It is clear that the ceasefire is getting more and more fragile.”