Kyiv, Ukraine - Ukrainian President Volodymyr Zelensky confirmed that a long-awaited counteroffensive against Russian forces is now underway.

"Counteroffensive and defensive actions are taking place in Ukraine: at which stage I will not talk in detail," Zelensky said on Saturday, commenting after Russian President Vladimir Putin claimed that Kyiv's operation was already failing.

Russia has reported thwarting Ukrainian attacks in the east and south.

"It's interesting what Putin said about our counteroffensive. It is important that Russia always feels this: that they do not have long left, in my opinion," Zelensky said.

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He added that he was in daily touch with military commanders, including armed forces chief Valery Zaluzhny, and "everyone is positive now – tell that to Putin!"

On Saturday, Kyiv's forces conducted counteroffensive operations in at least four front-line areas, according to the Washington DC-based think tank the Institute for the Study of War (ISW).

"Russian sources claimed that Ukrainian forces have tactical advantages in conducting assaults at night due to Western-provided equipment with superior night optics systems," ISW said.

To shore up that stockpile, Zelensky secured fresh pledges of military aid while meeting with visiting Canadian Prime Minister Justin Trudeau over the weekend.

Trudeau and Zelensky hugged and used each other's first names as the Canadian leader made his second unannounced visit to Kyiv since full-scale war broke out in February last year.

Canada, which hosts a large Ukrainian diaspora, has been one of Kyiv's key allies since the Russian invasion.

It has provided Ukraine with significant military aid, trained more than 36,000 soldiers and adopted sanctions against Moscow.

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On Saturday, the Canadian leader said he would provide Can$500 million (US$375 million) in fresh military assistance to Kyiv, and pledged Canada would be part of the multinational effort to train Ukraine's fighter pilots.

Trudeau also urged international organizations to come to the aid of those in Russian-occupied territories after flooding from the breached Kakhovka dam forced thousands to flee their homes and sparked fears of humanitarian and environmental disasters.

Ukraine accuses Russia of blowing up the dam on Tuesday, while Moscow says Kyiv fired on it.

When asked why he stopped short of blaming Russia for the dam collapse, Trudeau said: "I know there are investigators and there are a lot of questions and intelligence being analyzed on what exactly happened to have this dam collapse."

"But there is no doubt in my mind that absent Russia's invasion of last February, that dam would still be standing today."

Pledging 10 million Canadian dollars in flood relief, Trudeau also said Russia "will be held to account" for its actions in Ukraine.