Ukraine says it has retaken eight villages in the early stages of its most ambitious assault since Russia's full-scale invasion 16 months ago, but President Volodymyr Zelenskiy said this week that gains had been "slower than desired."

Addressing the pace of the Ukrainian advances, three senior officials on Friday sent the clearest signal so far that the main part of the counteroffensive has not yet begun.

"Offensive operations of the Armed Forces of Ukraine continue in a number of areas. Formation operations are underway to set up the battlefield," presidential adviser Mykhailo Podolyak said on Twitter.

"The counteroffensive is not a new season of a Netflix show. There is no need to expect action and buy popcorn."

Deputy Defence Minister Hanna Maliar said the "main events" of the counteroffensive were "ahead of us."

"And the main blow is still to come. Indeed, some of the reserves - these are staged things - will be activated later," Maliar told Ukrainian television.

Although the advances Ukraine has reported this month are its first substantial gains on the battlefield for seven months, Ukrainian forces have yet to push to the main defensive lines that Russia has had months to prepare.

"I want to say that our main force has not been engaged in fighting yet, and we are now searching, probing for weak places in the enemy defences. Everything is still ahead," British newspaper the Guardian quoted Oleksandr Syrskyi, the commander of Ukraine's ground forces, as saying in an interview.

'BALANCED APPROACH NEEDED'

Moscow has sought to portray the Ukrainian counteroffensive as a failure. It says Kyiv's forces have suffered heavy losses, while Ukraine says Russia has lost many soldiers in heavy fighting since the counterattack began.

Reuters is unable to verify the situation on the battlefield but has reached two of the villages recaptured by Kyiv.

Ukraine has prepared new military units for its long-awaited counteroffensive, including 12 new brigades, but only three of them have been seen in combat so far. It has also received an array of weapons from its Western allies to help it take back swathes of territory occupied by Russia.

Podolyak said that the time Ukraine had needed to convince its Western partners to provide the necessary weapons had given the Russian military the opportunity to dig in and strengthen their defence lines.

"Breaking the Russian front today requires a reasonable and balanced approach. The life of a soldier is the most important value for Ukraine today," he said.