The Obama administration, which has placed levied increasingly punitive economic sanctions on Russia over the Ukraine crisis, on Tuesday asserted that the Russians had sent new columns of tanks and armor across the border. The American ambassador to Kiev, Geoff Pyatt, said a "Russian-directed counteroffensive may be underway."
Russian forces have been trying to help the separatists break the siege of Luhansk and have been fighting to open a corridor to Donetsk from the Ukrainian-Russian border, Western officials say.
To the south, Russia has been backing a separatist push toward the southern town of Mariupol, a major port on the Azov Sea, according to Western and Ukrainian officials. The Russian aim, one Western official said, was to open a new front that would divert Ukrainian forces from Donetsk and Lukhansk and possibly seize an outlet to the sea in the event that Russia tries to establish a separatist enclave in the eastern Ukraine.
Some Western officials fear the move might even be a step in what they suspect is a broader Russian strategy to carve out a land link to Crimea, the strategic Ukrainian peninsula that Russia annexed in March, setting off Moscow's worst crisis with the West since the Cold War.
The Russian military's use of artillery from locations within Ukraine is of special concern to Western military officials, who say Russian artillery has already been used to shell Ukrainian forces near Luhansk. And along with the antiaircraft systems operated by separatists or Russian forces inside Ukraine, the artillery has the potential to alter the balance of power in the struggle for control of eastern Ukraine.
Russia has denied that it has intervened militarily in Ukraine and the separatists have asserted that they are using captured Ukrainian equipment. But American officials say they are confident that the Russian artillery, which is located in Ukraine's Krasnodon area, is Russia's since Ukrainian forces have not penetrated that deeply into that separatist controlled region. American officials also say the separatists have no experience in using such weaponry.
"We judge that self-propelled artillery is operated by Russians rather than separatists since no separatist training on this artillery has occurred to date," an Obama administration official said.
American officials say they also have imagery that shows the Russian military moving self-propelled artillery into position inside Ukraine last week and deploying them in firing position.
The Ukrainian retreat on Tuesday from the border area near Novoazovsk, came the same day that President Vladimir V. Putin of Russia, at a summit with his Ukrainian counterpart in Minsk, Belarus, continued to assert that Russia was a bystander. Mr. Putin called the Ukrainian insurgency an internal matter and that the Ukrainian government needed to negotiate a cease-fire.
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