俄罗斯在乌克兰的新举动挑战北约

LONDON— NATO, the United States and Ukraine say Russian forces are taking a more significant role in fighting in eastern Ukraine, bolstering separatist groups that have lost considerable ground to the government forces in recent weeks.

The cultural center of rebel-held Donetsk became a casualty of the fighting when it was hit by artillery shells. Ukrainian troops are pressing their offensive in the east, even amid reports of Russian forces joining separatists to rout other Ukrainian forces farther south.

Ukrainian President Petro Poroshenko says Russian troops have entered his country and the security situation has deteriorated. He wants a meeting of the U.N. Security Council.

NATO and U.S. officials agree that unmarked military vehicles, seen just inside Russian territory, are crossing the border to provide equipment, ammunition and perhaps 1,000 fighters to help the separatists.

The prime minister of the self-proclaimed Donetsk People’s Republic acknowledged the presence of as many as 4,000 Russians over the last several months, but said they are not regular military units. He claimed some of them are soldiers on vacation, but Ukrainian and Western officials say it is an organized invasion, designed not to look like one, similar to what Russia did in Crimea in February.

Ukrainian troops have been making significant gains against the separatists in recent weeks, apparently prompting the stronger Russian response. The separatists want independence.

But experts say Russian President Vladimir Putin, who met with Ukraine’s president on Tuesday, may be satisfied with some sort of long-term influence in Ukraine. Even that is too much for Poroshenko, but he may have a hard time convincing the West to help him prevent it, according to experts like former British and European Union official Nick Witney, now at the European Council on Foreign Relations.

“The way out is, in the short term a truce, a cease-fire, and ultimately some sort of political deal or understanding, which unfortunately will have to recognize that Russia does have more of an interest in Ukraine than we in the West do," he said.

At London’s Chatham House, senior analyst Xenia Wickett says the West is not likely to provide much help.

“NATO has actually kind of tripped over itself. By putting out very clear ‘Red Lines,’ it has actually got itself in a bind because Russia knows it can walk right up to those red lines but not cross them, without a NATO response," said Wickett.

Experts say the NATO summit in Britain next week could provide some clues about the alliance’s intentions regarding Ukraine, but no one expects dramatic action.