Fighting is raging at several points on the front line, Russian and U.S. officials say.
Ukrainian forces stepped up their artillery strikes and ground assaults in a flurry of offensive military activity that by Monday was raging along multiple sectors of the front line, American and Russian officials said.
Ukraine has remained silent about military operations after months of preparing for a major counteroffensive in the war. But the American officials, who spoke on the condition of anonymity, said the surge in attacks was a possible indication that Kyivs long-planned counteroffensive against Russian forces had begun.
The officials based their assessment in part on information from U.S. military satellites, which detected an uptick in action from Ukrainian military positions. The satellites have infrared capabilities to track artillery fire and missile launches.
One difficulty in determining the exact start of a counteroffensive, beyond Ukraines operational security measures, is that the fighting could well begin with feints or diversions that are hard to decipher.
The Russian Ministry of Defense announced on Monday that a major Ukrainian operation had begun at five locations along the front in one eastern region, Donetsk, and said it had repelled the assaults and inflicted casualties on the Ukrainian forces. Moscows reports could not be corroborated.