BRUSSELS/DONETSK (REUTERS) - President Vladimir Putin said on Monday that Russia is sending an aid convoy to eastern Ukraine despite urgent Western warnings against using humanitarian help as a pretext for an invasion.
With Ukraine reporting Russia has massed 45,000 troops on its border, Nato said there was a "high probability" that Moscow could intervene militarily in the country's east, where Kiev forces are closing in on pro-Russian separatists.
European Commission President Jose Manuel Barroso delivered a blunt message in a telephone call with Mr Putin on Monday.
"President Barroso warned against any unilateral military actions in Ukraine, under any pretext, including humanitarian," the Commission said in a statement.
The Kremlin, in its own account of the conversation, made clear that Moscow would indeed send help to largely Russian-speaking eastern Ukraine.
"It was noted that the Russian side, in collaboration with representatives of the International Committee of the Red Cross, is sending an aid convoy to Ukraine," the Kremlin statement said, without saying when the convoy was going.
UN agencies say more than 1,100 people have been killed including government forces, rebels and civilians in the four months since rebels seized territory in the east and Kiev launched its crackdown.
Ukrainian President Petro Poroshenko said he supported an aid mission but made clear it had to be an international effort under the aegis of the Red Cross, involving the European Union as well as Russia.
Mr Poroshenko said US President Barack Obama had also backed the international plan when they spoke on the telephone on Monday.
The International Committee of the Red Cross made no immediate comment, although last weekend it issued a statement acknowledging receipt of an offer from Russian Foreign Minister Sergei Lavrov about organising aid convoys.
The past week has seen increasingly urgent warnings from Kiev and Western countries that Moscow appeared to be planning an invasion.
Western countries say Mr Putin - who has whipped up the passions of Russians with a relentless nationalist campaign in state-controlled media since annexing Ukraine's Crimea peninsula in March - could invade to head off a humiliating rebel defeat.
Kiev said it was in the "final stages" of recapturing the eastern city of Donetsk - the main base of the separatist rebels - in a battle that could mark a turning point in a conflict that has caused the biggest confrontation between Russia and the West since the Cold War.
An industrial metropolis with a pre-war population of nearly one million, Donetsk rocked to the crash of shells and gunfire over the weekend, and heavy guns boomed through the night into Monday from the outskirts of the city.
Nato Secretary-General Anders Fogh Rasmussen said there was no sign Russia had withdrawn the troops it had massed at the Ukrainian frontier.
Asked in a Reuters interview how he rated the chances of Russian military intervention, Mr Rasmussen said: "There is a high probability."
"We see the Russians developing the narrative and the pretext for such an operation under the guise of a humanitarian operation, and we see a military build-up that could be used to conduct such illegal military operations in Ukraine," he said.
Nato believes any Russian humanitarian mission would be used as a pretext to save the rebels, who are fighting for control of two provinces under the banner of "New Russia", a term Mr Putin has used for southern and eastern Ukraine, where mostly Russian is spoken.
Ukraine appears to be pressing ahead with its offensive, undeterred by the presence of what Nato says are about 20,000 Russian troops massed on the nearby border for a potential ground invasion.
Kiev put the size of the Russian forces much higher. "As of 11 o'clock today, about 45,000 troops of the armed forces and internal forces of the Russian Federation are concentrated in border areas," Ukrainian military spokesman Andriy Lysenko told a briefing.
He said they were supported by 160 tanks, 1,360 armoured vehicles, 390 artillery systems, up to 150 Grad missile launchers, 192 fighter aircraft and 137 attack helicopters.
Kiev has said in recent days that it succeeded in using diplomacy to prevent Russia from launching a ground invasion to protect the rebels under the guise of a humanitarian mission. Moscow announced on Friday it was ending war games in the area.
Mr Lysenko said Ukrainian government forces had finally succeeded in cutting off the road between Donetsk and Luhansk, the other main rebel-held city, which is closer to the Russian border. Kiev and its Western allies say the route has been the principal means of supplying the rebels in Donetsk with weapons.
"The forces of the anti-terrorist operation are preparing for the final stage of liberating Donetsk," Mr Lysenko told Reuters. "Our forces have completely cut Donetsk off from Luhansk. We are working to liberate both cities, but it's better to liberate Donetsk first - it is more important."
The leader of the rebels in Donetsk, Mr Alexander Zakharchenko, a local man who took over the leadership from a Russian citizen last week, said the fighters were considering mounting a counter-attack against government forces in the next two or three days.
And a volunteer government fighter suggested claims that government forces were about to take Donetsk were inflated.
"Taking the town is an extremely complicated business and painful ... It will take, at the very least, several weeks," Mr Andriy Beletsky, commander of the so-called Azov battalion, told journalists.
Mr Lysenko said clashes took place in several parts of eastern Ukraine over the past 24 hours, with six Ukrainian service members killed and big losses to the rebel side. Rebel losses could not be independently confirmed.
Municipal authorities in Donetsk said artillery shelling knocked out power stations in the city and hit a high-security prison, killing one inmate and allowing more than 100 criminals to escape.
UN agencies say more than 1,100 people have been killed including government forces, rebels and civilians in the four months since rebels seized territory in the east and the government launched its crackdown. Kiev says 568 of its troops have been killed in combat.
Government forces have been advancing since June, pushing rebels into Donetsk and Luhansk, capitals of two provinces that the fighters have declared independent "people's republics".
Fighting in recent weeks has focused on the route linking the cities, near where Malaysia Airlines flight MH17 crashed in July, killing all 298 people on board. Washington says the plane was almost certainly shot down accidentally by rebels using an advanced Russian missile. Moscow denies this.
Fighting near the site forced the Netherlands to put off a mission to recover human remains and investigate the crash.
The rebels in eastern Ukraine have been led mostly by Russian citizens and they have heavy weaponry that Kiev and its allies say can only have come from Russia. Moscow denies aiding them.
Donetsk is facing an increasing shortage of food, water and fuel. Few people are on the streets, apart from groups of armed separatist fighters. There is relatively little traffic, with petrol in short supply. Those who have not left for the countryside are staying indoors. Banks are closed and pensions and social allowances are not being paid.
Shelling on Monday from the direction of the international airport and the suburb of Yasynuvata to the north knocked out a string of power stations, the municipal authority said.