Ukrainian servicemen ride in an armoured vehicle near Debaltseve, in the Donetsk region, yesterday. Photo: Reuters

Ukraine called yesterday for full membership in Nato, its strongest plea yet for Western military help after accusing Russia of sending in armoured columns that have driven back its forces on behalf of pro-Moscow rebels.

Russian President Vladimir Putin, defiant as ever, compared Kiev’s drive to regain control of its rebellious eastern cities to the Nazi invasion of the Soviet Union in World War II. He announced that rebels had succeeded in halting it.

The past 72 hours have seen pro-Russian rebels suddenly open a new front and push Ukrainian troops out of a key town in strategic coastal territory along the Sea of Azov. Kiev and Western countries say the reversal was the result of the arrival of armoured columns of Russian troops, sent by Putin to prop up a rebellion that would otherwise have been near collapse.

Full Ukrainian membership of Nato, complete with the protection of a mutual defence pact with the US, is still an unlikely prospect. But by announcing it is now seeking to join the alliance, Kiev has put more pressure on the West to find ways to protect it. Nato holds a summit next week in Wales. Nato’s Secretary General Anders Fogh Rasmussen said he respected Ukraine’s right to seek alliances.

“Despite Moscow’s hollow denials, it is now clear that Russian troops and equipment have illegally crossed the border into eastern and southeastern Ukraine,” Rasmussen said. “This is not an isolated action, but part of a dangerous pattern over many months to destabilise Ukraine as a sovereign nation.”

Kiev said it was rallying to defend the port of Mariupol, the next big city in the path of the pro-Russian advance in the southeast.

“Fortifications are being built. Local people are coming out to help our troops, to stop the city being taken. We are ready to repel any offensive on Mariupol,” military spokesman Andriy Lysenko said.

So far, the West had made clear it is not prepared to fight to protect Ukraine but is instead relying on economic sanctions, first imposed after Russia annexed Ukraine’s Crimea peninsula in March. But those sanctions seem to have done little to deter Putin.

European foreign ministers met in Milan yesterday. They made clear the bloc will discuss further economic sanctions against Moscow.

In 2008 Nato denied Ukraine and Georgia a fast track towards membership. Russia invaded Georgia a few months later.