NOVOAZOVSK (TIP): Tanks, artillery and infantry have crossed from Russia into an unbreached part of eastern Ukraine in recent days, attacking Ukrainian forces and causing panic and wholesale retreat not only in this small border town but also a wide section of territory, in what Ukrainian and Western military officials described on August 27 as a stealth invasion.

The attacks outside this city and in an area to the north essentially have opened a new, third front in the war in eastern Ukraine between government forces and pro-Russian separatists, along with the fighting outside the cities of Donetsk and Luhansk. Exhausted, filthy and dismayed, Ukrainian soldiers staggering out of Novoazovsk for safer territory said Tuesday they were cannon fodder for the forces coming from Russia. As they spoke, tank shells whistled in from the east and exploded nearby. Some of the retreating Ukrainian soldiers appeared unwilling to fight.

The commander of their unit, part of the Ninth Brigade from Vinnytsia, in western Ukraine, barked at the men to turn around, to no effect. “All right,” the commander said. “Anybody who refuses to fight, sit apart from the others.” Eleven men did, while the others returned to the city. Some troops were in a full, chaotic retreat: a city-busload of them careened past on the highway headed west, purple curtains flapping through windows shot out by gunfire.

A Ukrainian military spokesman said Wednesday the army still controlled Novoazvosk but that 13 soldiers had died in the fighting. The behavior of the Ukrainian forces corroborated assertions by Western and Ukrainian officials that Russia, despite its strenuous denials, is orchestrating a new counteroffensive to help the besieged separatists of the Donetsk People’s Republic, who have been reeling from aggressive Ukrainian military advances in recent weeks. “Russia is clearly trying to put its finger on the scale to tip things back in favor of its proxies,” said a senior American official.

“Artillery barrages and other Russian military actions have taken their toll on the Ukrainian military.” The Obama administration, which has placed increasingly punitive economic sanctions on Russia because of the Ukraine crisis, asserted over the past few days that the Russians had sent new columns of tanks and armor across the border. “These incursions indicate a Russiandirected counteroffensive is likely underway,” Jen Psaki, the state department spokeswoman, said Wednesday.

At the department’s daily briefing in Washington, Ms. 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 territory. Ms. Psaki apparently was referring to videos of captured Russian soldiers, distributed by Ukraine’s government on Tuesday, that directly challenged President Vladimir Putin’s assertions that Russia is a mere bystander in the conflict. The videos were publicized just as Putin was meeting with his Ukraine 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. To the south, Russia has been backing a separatist push toward the southern town of Mariupol, a major port on the Sea of Azov, according to Western and Ukrainian officials.

The Russian aim, one Western official said, is to open a new front that would divert Ukrainian forces from Donetsk and Luhansk and to possibly seize an outlet to the sea in the event that Russia tries to establish a separatist enclave in eastern Ukraine.