Fighting in eastern Ukraine has "somewhat intensified" as Ukrainian and Russian forces clash in at least three areas on the eastern front, a senior Ukrainian defense official said on Sunday.

Kyiv launched a counteroffensive last month in a bid to retake Russian-occupied territory and seize the initiative in Russia's full-scale invasion, now in its 17th month.

Deputy Defense Minister Hanna Maliar said on Telegram that Russian forces have been attacking in the direction of Kupyansk in the Kharkiv region for two successive days.

"We are on the defensive," Maliar wrote. "There are fierce battles, the positions of both sides change dynamically several times a day."

Maliar also said the two armies were pummeling one another around the ruined city of Bakhmut but that Ukrainian forces were "gradually moving forward" along its southern flank.

She added that Kyiv's troops were also fending off Russian attacks near Avdiivka and Maryinka.

A spokesman for the military's southern command said in a separate statement on Sunday that Ukrainian forces had advanced more than a kilometer in one part of the southern front.

Kyiv has made incremental gains in parts of the east and south since launching its long-awaited counteroffensive.

Russian President Vladimir Putin said in an interview with state television, part of which was released on Sunday, that the operation was "not succeeding" and that attempts to break through Russian defenses had failed.