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Russia, which has denied it is helping the insurgents, did not acknowledge the military movements. But the Russians have signaled that they would not countenance a defeat of an insurgency in the heavily Russian eastern part of Ukraine, which would amount to a significant domestic political setback for President Vladimir Putin of Russia in his increasingly fractious relationship with the United States and its European allies.

“Russia is clearly trying to put its finger on the scale to tip things back in favor of its proxies,” a senior US official said. “Artillery barrages and other Russian military actions have taken their toll on the Ukrainian military.”

The Russian military movements carried the potential to poison any hope that a halt to the five-month-old conflict was any closer, one day after the presidents of both countries, at a meeting in Belarus, outwardly professed their desire for a solution. Russia’s behavior also raised the possibility of punitive new Western economic sanctions as a reprisal.

Andriy Lysenko, a spokesman for the Ukrainian military in Kyiv, said the Russian armored column entered the town of Amvrosiyivka, south of Donetsk, expanding what Western and Ukrainian officials have described as one of the main fronts in a multipronged, Russia-directed counteroffensive. This week, Ukraine accused Russia of sending an armored column toward the coastal city of Mariupol, far from the fighting around Luhansk and Donetsk, with the aim of diverting Ukrainian forces to deal with that new threat.

The Obama administration accused Russia of lying about its intentions, while European officials angrily demanded answers from the Kremlin. Chancellor Angela Merkel of Germany, who perhaps has the most cordial relationship with Putin, telephoned him Wednesday to request an explanation, her office said.

Evidence that Russia was seeking to change the course of the conflict was abundant this week in the small southeast border town of Novoazovsk, where Ukrainian forces beat a nervous retreat on Tuesday, under attack from what fleeing soldiers described as columns of tanks, artillery, and troops coming across the border.

More fighting and shelling punctuated the area around the town Wednesday, although it was unclear whether the assailants were Russian forces or members of the Donetsk People’s Republic, the name the separatists have given themselves.

The Obama administration, which has imposed increasingly punitive economic sanctions on Russia because of the Ukraine crisis, has asserted over the past week that the Russians had moved artillery, air-defense systems, and armor to help the separatists in Donetsk and Luhansk.

“These incursions indicate a Russian-directed counteroffensive is likely underway,” Jen Psaki, a State Department spokeswoman, said Wednesday.

At the department’s daily briefing in Washington, Psaki also criticized what she called the Russian government’s unwillingness to tell the truth that its military had sent soldiers as deep as 30 miles inside Ukraine’s territory.

Psaki apparently was referring to videos of captured Russian soldiers, distributed by the Ukrainian government on Tuesday, that directly challenged Putin’s assertions that Russia is a mere bystander in the conflict. The videos were publicized just as Putin was meeting with his Ukrainian counterpart, Petro Poroshenko, in Belarus.

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.

The Russian military’s use of artillery from 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.