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 Kyiv’s long-planned counteroffensive against Russian forces had begun.

The officials based their assessment in part on information from US 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 Ukraine’s operational security measures, is that the fighting could well begin with feints or diversions that are hard to decipher.

The Russian ministry of defence 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. Moscow’s reports could not be corroborated.

Russian bloggers affiliated with the Russian military went further, saying Ukraine had advanced in some areas and that an even stronger attack had begun early on Monday morning near the town of Velyka Novosilka.

There was no immediate response from the Ukrainian authorities to the Russian claims.

Buttressing the reports that the counteroffensive may have started, American military analysts said they believed Ukrainian units had begun an initial thrust to determine the position and strength of Russia’s forces. Such moves would be a traditional tactic of the kind Americans have been training Ukrainian forces to undertake.

Much is riding on Ukraine’s counteroffensive to reclaim territory lost to Russia and free Ukrainians from living under occupation. Success could help support Kyiv’s push for longer-term commitments for additional military aid and security guarantees from the West. It also could strengthen the hand of President Volodymyr Zelensky in any peace talks with Russia.

Failure could complicate Ukraine’s ability to get further assistance from the West and make Kyiv’s push for additional security guarantees at the Nato summit this summer more difficult.