The latest alleged movement of soldiers, rocket launchers, tanks and missiles from Russia into rebel-held eastern Ukraine would fit a familiar pattern in the country's separatist crisis, but for a few worrisome details.

The unmarked convoys spotted near the Russian-backed breakaway "republics" of Donetsk and Luhansk last week appear to contain advanced weapon systems and radar equipment never before seen in the months-long conflict — supplies some analysts speculate could be used to launch a lightning offensive by the separatists. And whereas past alleged incursions near the porous Russia-Ukraine border have occurred while fighting has raged, the rebels are currently supposed to be observing a cease-fire with Ukrainian forces that Moscow helped broker, even if it has been largely ignored by both sides.

Though Russia has dismissed the latest charges as "hot air," Western powers have little patience for such denials. There's wide agreement among analysts that Russian President Vladimir Putin seeks to destabilize his southwestern neighbor, where the newly elected government in Kiev is desperately trying to foster closer ties with NATO and the European Union.

But the timing and nature of these latest reinforcements has left diplomats scrambling to decipher just how far Putin might be willing to go to achieve that end.

Writing in The New Republic, Julia Ioffe and Linda Kinstler said last week’s military buildup has echoes of Russia's annexation of Crimea in March. There, Putin denied that a sudden incursion of unmarked troops into the peninsula had anything to do with Moscow until after a public referendum voted in favor of secession. A similar plot could be in store for the largely Russian-speaking Donbass region of eastern Ukraine, Ioffe and Kinstler suggested. Another theory holds that Russia is arming its rebel allies for an even deeper offensive aimed at opening up a land bridge from the separatist east to isolated Crimea, which would facilitate the transport of supplies to sustain Russia’s newest citizens.

But others say that despite Putin’s insistence on referring to the Donbass region as Novorossiya — New Russia — a land grab in eastern Ukraine would actually run counter to Russia’s objectives. They note that if Moscow were to slice off the largest pro-Russian sector of Ukraine, it would leave behind a state — albeit a smaller, weaker one — that is almost entirely pro-Western.

“I see Donbass as very different from Crimea,” said Dmitri Trenin, director of the Carnegie Moscow Center. The peoples’ republics “have been important for Putin as a foot in the door in his policy of guaranteeing Russian interests in Ukraine as a whole, such as neutrality between NATO and Russia. Annexation serves no useful Russian interest at this point.”

Most analysts have ruled out that Putin would ever launch a formal invasion of separatist-held Ukraine, even though he has said that step could be justified on humanitarian grounds and maintains tens of thousands of regular Russian soldiers just across the border. Recent polling indicates that Russians generally approve of their president’s confrontation with the West over Ukraine and of the “volunteer” Russian military units that Moscow says are fighting on behalf of their separatist brethren. But a Levada Tsenter poll found that 68 percent of Russians were opposed to sending troops into Ukraine, and there is rising concern over the toll Western sanctions are taking on the Russian economy.