President Petro Poroshenko said on Wednesday more than 9,000 Russian troops were backing Ukrainian separatist fighters that Moscow will be under pressure to reel in at high-stakes peace talks in Berlin.

Poroshenko's claims followed days of heavy fighting that has left an already shaky September truce in tatters and forced the pro-Western leader to cut short his visit to the World Economic Forum in Davos.

But the 49-year-old former chocolate baron was greeted warmly at the Swiss forum. Repeated rounds of applause broke out during an emotional address that saw Poroshenko hold up a fragment of a bus in which 13 civilians died after being hit by an alleged rebel-fired rocket last week.

"Terror is not the problem of Ukraine, and even not the problem of Europe, this is a global problem," Poroshenko told the high-powered audience in English.

The east of Ukraine "is the line of the front where we are fighting not only for our territorial integrity, sovereignty and independence, but for European security, for European values."

More clashes on Wednesday killed at least six people across separatist-controlled regions of the ex-Soviet republic's industrial southeast.

Poroshenko said that a recent upsurge in attacks that followed a nearly month-long lull was prompted by a new infusion of Russian forces and advanced equipment designed to propel a new separatist land grab.

"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," he claimed.

Ukraine on Tuesday alleged that Russian regular forces attacked its troops in Lugansk after crossing over into the separatist region the day before.

Moscow strongly denies supporting the insurgents despite NATO satellite imagery purporting to show its forces' presence in Ukraine -- photographs Russia claims were either doctored or misinterpreted by the Western military bloc.

"As for the charges of us supplying (the rebels with) troops and weapons -- we hear this all the time," Russian Foreign Minister Sergei Lavrov told reporters in Moscow.

"And each time, I say: if you are so sure about this, show us the proof," Lavrov added. "But the proof -- they either do not want to present it or simply cannot."

A top U.S. general -- in Kiev to help bolster Ukraine's NATO ties -- said evidence of Russia doubling the amount of modern military equipment available to the militias in recent weeks was "very clear".

"If you don't believe that that's being provided by Russia, then you do not want to believe," U.S. Army Europe commander Ben Hodges said.

Russia's denials are also unlikely to keep the Ukrainian charges from overshadowing crisis talks between the two countries' foreign ministers and the top diplomats of Germany and France on Berlin due to begin late Wednesday.

Ukraine's severe financial crisis caused by extra war spending and the eastern region's industrial collapse has further complicated the situation, with Kiev requiring billions of dollars in help from world lenders.

Compounding the problem last year was a months-long cut off from Russian gas supplies in a dispute that stemmed from a disagreement over prices.

Kiev accused Moscow of using gas as a means of "economic aggression" and refused to pay the higher rates Russia slapped on Ukraine in the wake of last February's ouster of a Kremlin-backed president.

Poroshenko said Wednesday that Ukraine in the past year halved the amount of gas it imported from Russia and insisted that this trend would continue in the months to come.

He vowed to completely break Ukraine's expensive energy dependence on Russia by 2017 with the help of a transition to western European imports and his country's own shale gas supplies.

"I am absolutely sure that in two years' time, we will be absolutely energy independent from Russia," said Poroshenko.

Moscow and Kiev have traded blame for the recent wave of fighting that has centered around Donetsk's ruined airport.

The flare-up coincided with attempts by both sides to establish a demarcation line between their armies that would define the extent of rebel-controlled territory.

Two top Western diplomats in Kiev said they believed the pro-Russian militias had made significant progress on the ground in recent days.

One of them said the rebel offensive appeared to be aimed at undermining European peace efforts in order to win more ground before a final partition agreement is reached.