Ukrainian president has said more than 9,000 Russian troops are backing Ukrainian separatists, while appealing to the International Monetary Fund chief for extra financial help for his country.
Petro Poroshenko, speaking at the World Economic Forum on Wednesday, said Russian soldiers were backed by a range of heavy weapons, including tanks, heavy artillery and armoured vehicles.
"If this is not aggression, what is aggression?" he said.
Sergei Lavrov, Russia's foreign minister, tried to fend off the accusations, saying he hoped for progress at talks on the conflict on Wednesday despite the renewed fighting.
"If you allege this so confidently, present the facts. But nobody can present the facts, or doesn't want to," Lavrov said before heading to the peace talks in Berlin with the foreign ministers of Ukraine, Germany and France.
In Brussels, Jens Stoltenberg, NATO secretary-general, said that the Western alliance had monitored Russian troops in Ukraine for several months and seen an increase lately in numbers of tanks, artillery pieces and other heavy equipment.
Poroshenko called on Russia to comply with a peace plan reached in Minsk, Belarus, last September between Ukraine, Russia and pro-Russian separatist leaders to end the conflict.
"The solution is very simple - stop supplying weapons ... withdraw the troops and close the border," he said.
"If you want to discuss something different, it means you are not for peace, you are for war."
The Minsk plan provides for a ceasefire and the withdrawal of foreign fighters and military equipment from Ukraine. But the ceasefire has been very shaky from the start and hundreds of people have died since September in clashes Kiev says have involved regular Russian troops.
Ukraine's economy has been pushed close to bankruptcy by the war with the rebels, and economists have warned of debt default if an existing IMF loan programme is not expanded to make up for a estimated $15bn funding gap.
Poroshenko told Christine Lagarde, IMF managing director, at a meeting in Davos that his government had appealed for more funding.
"President Poroshenko informed me today that the Ukrainian authorities have requested a multi-year arrangement with the Fund ... to replace the existing Stand-By Arrangement," she said in a statement.
Lagarde said the IMF board would discuss the request, and made clear that Ukraine would have to make broad and deep economic reforms in return for any deal.
However, she said: "I will submit it to the board which will convene as soon as possible. I will propose to support it."