Ukrainian President Petro Poroshenko said on Wednesday tht Russia had 9,000 troops on Ukrainian soil supporting separatist forces there and he called on Moscow to withdraw them.
In a speech to the World Economic Forum in Davos, Poroshenko said the Russian troops were backed by a range of heavy weapons, including tanks, heavy artillery and armoured vehicles.
"If this is not aggression, what is aggression?" he asked.
Poroshenko said Moscow must comply with a peace plan agreed in Minsk, Belarus, last September between Ukraine, Russia and separatist leaders to end a conflict in which more than 4,800 people have died since it flared up last April.
"Our approach is very simple we have nothing to negotiate ... We have the Minsk format and we need immediately just a ceasefire and the withdrawal of heavy artilllery and weapons and tanks from the touchline," Poroshenko said.
"The solution is very simple - stop supplying weapons ... withdraw the troops and close the border. Very simple peace plan. If you want to discuss something different, it means you are not for peace, you are for war," he said.
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.
Moscow denies its forces are directly involved in the conflict.
Poroshenko also called on Russia to cooperate in closing their long, but porous, joint border.
To reinforce his message about what he branded "terrorist attacks" in Ukraine, he held up a large piece of yellow metal sheeting pockmarked with holes, saying it was part of a bus hit in a rebel missile attack.
Thirteen civilians died and many others were injured in the attack on the bus, which was struck when it passed a Ukrainian army checkpoint in Volnovakha on Jan. 13.
"I have here part of the Volnovakha bus with the hits of the fragments of the Russian missiles which are hitting my people. It is a symbol of the terrorist attack against my country," Poroshenko said.