Fighting has been going on in the Russian border region of Kursk since August of this year, after a sudden incursion of Ukrainian units, while Russian forces are trying to expel them.
Ukrainian President Vladimir Zelensky claims that Moscow has gathered 50.000 soldiers there.
In a regular address, Zelensky said that the operation of Ukrainian units in the Kursk region "restricts Russia in its attacks inside Ukraine."
Zelenskiy has repeatedly described the surprise incursion of Ukrainian forces into Kursk as a strategic move to slow the Russian offensive in Ukraine, although some Western allies are suspicious.
According to the US Institute for the Study of War, Russia had 11.000 troops in Kursk when Ukraine launched its surprise incursion in early August.
However, in the report New York Times it is suggested that Moscow was able to build up troops in Kursk without having to withdraw troops from Ukraine.
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"Our people are holding back... 50.000 members of the occupying army who, due to the Kursk operation, cannot be deployed to other Russian offensive lines on our territory," said the Ukrainian president.
Zelensky added that he was informed about the situation on that front by the Supreme Commander of the Ukrainian Army, General Alexander Sirsky, who said earlier on Monday that he had inspected the Ukrainian units deployed in Kursk.
If Ukrainian forces had not entered Kursk, "tens of thousands of enemies from the best Russian units would have stormed" Ukrainian positions in the Donetsk region, Sirski said.
Donetsk has been a key battleground since the conflict broke out a decade ago.
Fierce fighting continues in that region in eastern Ukraine, and on Monday the two sides accused each other of damaging a dam near the Ukrainian-controlled city of Kurakhovo.
Russian troops have been slowly advancing in the region for months the key city of Pokrovsk - the main supply hub for the Ukrainian forces.
Citing unnamed American and Ukrainian officials, the New York Times estimates that around 50.000 Russian and North Korean troops are stationed in the Kursk region ready to counterattack Ukrainian forces.
"A new US assessment concludes that Russia has been building up forces without having to withdraw troops from eastern Ukraine, its main military objective, allowing it to apply pressure on multiple fronts simultaneously," the paper said.
The goal of the operation, which could begin in the coming days, will be to regain control over the entire Kursk region, adds the New York Times.
Both Ukraine and the US say more than 10.000 North Korean troops have been sent to Russia.
Moscow neither confirms nor denies that troops from North Korea, a close ally from Soviet times, are in Kursk.
During the visit of the Russian president to Pyongyang in June this year, North Korean leader Kim Jong Un and Vladimir Putin achieved an agreement on mutual assistance in case of aggression against one of those two countries.
Both presidents then ratified the agreement.
Since Russia began its invasion of Ukraine in February 2022, North Korea and Russia, which are under Western sanctions, have grown closer.
The US has repeatedly accused Pyongyang of sending huge amounts of military equipment to Russia, including ballistic missiles and launchers.
NATO Secretary General Mark Rutte recently suggested that Pyongyang is receiving military technology and other support from Moscow to help evade international sanctions.
Meanwhile, the Kremlin denied media reports that the winner of the US presidential election, Donald Trump, had a telephone conversation with Putin.
On Sunday, the Washington Post published information about the telephone conversation between Trump and Putin, which allegedly took place on Thursday.
The same newspaper stated that Trump allegedly warned the Russian president not to escalate the war in Ukraine and mentioned America's large military presence in Europe.
A Kremlin spokesman said such reports were "pure fiction," while Trump's team told the BBC they would not comment on the president-elect's "private calls."
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