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* NATO chief sees Russia developing pretext for intervention

* Government forces say getting close to capturing city

* But rebels still control swathes of territory in east

* Shelling hits prison in Donetsk, knocks out power stations

* City is short of food, electricity, water

By Adrian Croft and Sergei Karpukhin

BRUSSELS/DONETSK, Aug 11 (Reuters) - NATO said on Monday there was a "high probability" that Russia could launch an invasion of eastern Ukraine under the guise of a humanitarian operation, but a defiant President Vladimir Putin said Moscow was sending an aid convoy into the country.

With Ukraine reporting Russia has massed 45,000 troops on its border, European Commission President Jose Manuel Barroso also warned Putin against any military intervention in the country.

The past week has seen increasingly urgent warnings from Kiev and Western countries that Putin appeared to be planning an invasion.

Western countries said 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.

The European Union chief delivered his message in a telephone call to Putin. "President Barroso warned against any unilateral military actions in Ukraine, under any pretext, including humanitarian," the Commission said in a statement.

However, the Kremlin made clear that Moscow, which has expressed its concern about Russian speakers in eastern Ukraine as Kiev forces try to crush pro-Russian rebels, would indeed send aid.

"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 said in a statement on the conversation between the two leaders.

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 1 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NATO Secretary-General Anders Fogh Rasmussen said there was no sign Russia had withdrawn the troops it had massed at the Ukrainian frontier.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, 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.

HUMANITARIAN PRETEXT

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 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.

Ukrainian President Petro Poroshenko has himself said he is ready to support a humanitarian mission for Luhansk but only as long as it is sponsored by the international community. His administration said on Monday that Barroso, in a phone call with Poroshenko, had said the EU was ready to earmark 2.5 million euros for such a mission.

CITIES "CUT OFF"

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," 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, 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," said Andriy Beletsky, commander of the so-called Azov battalion told journalists.

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 a 100 criminals to escape.

U.N. 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. (Additional reporting by Richard Balmforth, Pavel Polityuk and Natalia Zinets in Kiev, Alexei Anishchuk and Lina Kushch in Donetsk, Katya Golubkova in Moscow and Barbara Lewis in Brussels; Writing by Richard Balmforth and Peter Graff; Editing by Peter Millership and Will Waterman)

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