Russia has 9,000 troops in Ukraine - President Poroshenko
Russia has more than 9,000 soldiers and 500 tanks, heavy artillery and armoured personnel carriers in eastern Ukraine, President Petro Poroshenko has said.
He urged Russia to withdraw its troops and comply with a ceasefire plan, amid escalating fighting between Ukrainian troops and rebels in the east.
Russia has repeatedly denied claims its soldiers are fighting with the rebels in the Donetsk and Luhansk regions.
Talks on de-escalating the crisis are now under way in Berlin.
Foreign ministers from Ukraine, Russia, France and Germany were expected to take part in the meeting in the German capital.
More than 4,800 people have been killed and some 1.2 million have fled since rebels took control of parts of Luhansk and Donetsk regions in April.
This followed Russia's annexation of Ukraine's Crimea peninsula in March.
Speaking at the World Economic Forum in Davos, Switzerland, President Poroshenko said the Russian troops in eastern Ukraine were backed by heavy weapons including tanks and artillery systems.
"If this is not aggression, what is aggression?" he asked.
Mr Poroshenko again called on Russia to comply with the ceasefire agreement reached in September in Minsk, Belarus.
That deal envisages the pullout of heavy weapons by both sides from the line of separation and the exchange of prisoners. It also stipulates that control of the Ukrainian-Russian border, parts of which are currently held by pro-Russian rebels, would be returned to Ukraine's authorities.
Mr Poroshenko is now cutting short his Davos visit and returning to Kiev in view of the worsening situation in eastern Ukraine.
Earlier on Wednesday, Russian Foreign Minister Sergei Lavrov admitted that the truce deal was failing because the line of separation was not being respected.
Mr Lavrov said he would be "pushing for an immediate ceasefire" in Berlin as well as the withdrawal of heavy weaponry from that line.
Russia had done its "utmost" to resolve the conflict and "maintain the integrity of Ukraine", he said, adding that there was no evidence of Russian soldiers or weapons crossing the border.
Referring to Western sanctions against Russia over its alleged support for the rebels, Mr Lavrov said all attempts to isolate Russia would fail.
Pro-Russian separatists have fought Ukrainian forces for control of the ruined airport at Donetsk and there have been fierce clashes at two checkpoints near the town of Slovyanoserbsk, north-west of Luhansk city.
On Tuesday, Ukraine alleged that "regular military formations" of Russian troops had attacked the checkpoints, although there has been no independent confirmation.
Ukrainian military officials also said two battalion groups, both of around 400 men, had crossed into Ukraine from Russia on Monday - a claim rejected by Moscow as "hallucinations about a Russian invasion".
Russia has repeatedly rejected accusations by Ukraine and the West that it has been sending its troops into Ukraine and arming the rebels.
However, Moscow acknowledges that Russian "volunteers" are fighting for the separatists.
On Wednesday, five civilians were killed and at least 30 wounded in shelling of several districts of the rebel stronghold of Donetsk, the local authority said. The city's Kyivskiy and Kuibyshivskiy areas were among those worst hit.
Meanwhile, Ukrainian Prime Minister Arseniy Yatsenyuk said a new law to increase the size of Ukraine's army to 250,000 personnel had been delivered to parliament on Wednesday. This signifies a rise of some 68,000 people, according to government figures.
Ukrainian foreign ministry spokesman Yevhen Perebyinis said on Wednesday that Russia had broken the Minsk agreements "the first day after they were signed".
He said rebels had gained 500 sq km (193 sq miles) in territory since then, he said.
"Now they want the airport, and Russian armed forces are trying to broaden that territory."
Geoffrey Pyatt, the US ambassador to Ukraine, told the BBC the recent escalation in fighting constituted a "turning point" in the conflict.
"This recent reignition of the crisis is a consequence of actions that have been taken by the Russian government - and what's going to deescalate the crisis is the actions that Russia is going to take to stop the transfer of weapons and heavy equipment and fighters across the border."
"Ukraine is the victim at this stage," he added.