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The estimates that Russian casualties in Ukraine so far have reached around 15,000 killed and perhaps 45,000 wounded, Director William Burns said on Wednesday, adding that Ukraine has also endured significant casualties.

Nearly five months since President Vladimir Putin ordered an invasion of Russia's neighbour, its forces are grinding through the Donbas region of eastern and occupy around a fifth of the country.

Burns, speaking at the Aspen Security Forum in Colorado, said those gains have come at great cost.

CIA Director Burns estimates 15000 Russians KIA in Ukraine, 45,000 wounded. Says Ukrainians also suffer similar but likely less casualties

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"The latest estimates from the US intelligence community would be something in the vicinity of 15,000 (Russian forces) killed and maybe three times that wounded. So a quite significant set of losses," Burns said.

"And, the Ukrainians have suffered as well - probably a little less than that. But, you know, significant casualties."

Russia classifies military deaths as state secrets even in times of peace and has not updated its official casualty figures frequently during the war. On March 25 it said 1,351 Russian soldiers had been killed.

The Kyiv government said in June that 100 to 200 Ukrainian troops were being killed per day.

On Wednesday, Russian Foreign Minister Sergei Lavrov said Moscow's military "tasks" in Ukraine now went beyond the Donbas, in the clearest acknowledgment yet that it has expanded its war goals.

But Burns said that at least for now the Russian military's concentration of forces in the Donbas suggested they had learned hard lessons from failures at the start of the campaign, where Moscow stalled in its assault on Kyiv.

"In a way, what the Russian military has done is retreat to a more comfortable way of war, in a sense, by using their advantages and long range firepower to stand off and effectively destroy Ukrainian targets and to compensate for the weaknesses in manpower that they still experience," Burns said.

In recent days, Ukrainian President Volodymyr Zelenskiy as carried out Kyiv's biggest internal purge of government officials in the war, citing their failure to root out Russian spies and announcing scores of treason cases.

The United States has provided Ukraine vast amounts of intelligence to help guide its battlefield decisions, raising questions about whether the CIA and the Pentagon should be concerned about Russian infiltration.

Burns, however, appeared to play down such concerns.

"We're confident that the partnerships we built are effective ones," Burns said.

"And we're sharing quite significant amounts of intelligence with the Ukrainian services and with the Ukrainian leadership that they're putting to a very effective use."Ìý