KIEV (Reuters) - Ukraine called on Friday for full membership in NATO, its strongest plea yet for Western military help, after accusing Russia of sending in armored 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 Two. He announced that rebels had succeeded in halting it, and proposed that they now permit surrounded Ukrainian troops to retreat.

Speaking to young people at a summer camp, Putin told his countrymen they must be "ready to repel any aggression towards Russia.” He described Ukrainians and Russians as "practically one people,” language that Ukrainians say dismisses the very existence of their thousand-year-old nation.

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 armored columns of Russian troops, sent by Putin to prop up a rebellion that would otherwise have been near collapse.

Rebels said they would accept Putin’s proposal to allow Kiev forces, who they say are surrounded, to retreat, provided the government forces turn over weapons and armor. Kiev said that only proved that the fighters were doing Moscow’s bidding.

Russia drew a fresh rebuke from French Foreign Minister Laurent Fabius, who told French television station France 24 that Russia could face more sanctions from the European Union.

"When one country sends military forces into another country without the agreement and against the will of another country, that is called an intervention and is clearly unacceptable,” he said.

In Berlin, German Foreign Minister Frank-Walter Steinmeier said after speaking with his Ukrainian counterpart: "The border violations we are seeing – yesterday and even more so the day before yesterday – make us fear that the situation is increasingly getting out of control.”

In Washington, White House spokesman Josh Earnest said Russia’s footprint was undeniable in Ukraine.

"We have regularly marshalled evidence to indicate what exactly is happening, despite the protestations of the Russian government that for some reason would have us all believe otherwise,” he said. "The fact is, those denials are completely without any credibility, and, you know, we’ve been pretty candid about that.”