Ukrainian President Volodymyr Zelensky has said his military's ongoing incursion into Russia's Kursk region is now holding down 50,000 Russian troops.
In his daily address to the nation, Zelensky said the operation was reducing Moscow's ability to attack inside Ukraine itself. The president has long cited this as the goal of the offensive, despite scepticism from some Western allies.
According to the Institute for the Study of War, a US non-profit, Russia had 11,000 troops in Kursk when Ukraine began its shock incursion in early August.
However, a report in the New York Times suggests Moscow has achieved its troop build-up in Kursk without any need to pull its soldiers out of Ukraine.
In his speech, Zelensky said he had been briefed by his Сommander-in-Сhief, Gen Oleksandr Syrskyy, who announced earlier on Monday that he had carried out an inspection of Ukrainian units deployed in Kursk.
"Our men are holding back... 50,000 of the occupier's army personnel who, due to the Kursk operation, cannot be deployed to other Russian offensive directions on our territory," the Ukrainian president said.
Gen Syrskyy said separately that were it not for Ukraine's forces inside Kursk, "tens of thousands of enemies from the best Russian assault units would have been storming" Ukrainian positions in Donetsk region, a key battleground since the conflict erupted a decade ago.
Fighting rages on in Donetsk, where the two sides accused each other on Monday of damaging a dam near the Ukrainian-held town of Kurakhove. Russian troops have been slowly advancing in the region for months towards the key city of Pokrovsk - a major supply hub for Ukrainian forces.
The New York Times, which quotes both US and Ukrainian unnamed officials, puts the number of Russian and North Korean troops being readied for the reported counter-offensive in Kursk at 50,000.
"A new US assessment concludes that Russia has massed the force without having to pull soldiers out of Ukraine’s east - its main battlefield priority - allowing Moscow to press on multiple fronts simultaneously," the paper says.
Both Ukraine and the US say that more than 10,000 North Korean soldiers have been sent to Russia. Moscow neither confirms nor denies that troops from North Korea, a close ally since Soviet times, are in Kursk.
In North Korea itself, it was announced that its leader, Kim Jong un, had signed a decree ratifying a mutual defence treaty with Russia, which was approved in June at a summit in Moscow with Russian President Vladimir Putin.
North Korea and Russia have grown increasingly close since Moscow found itself largely internationally isolated after its February 2022 invasion of Ukraine. The US has repeatedly accused Pyongyang of sending vast amounts of military hardware to Russia, including ballistic missiles and launchers.
Nato Secretary General Mark Rutte recently suggested that Pyongyang was receiving military technology and other support from Moscow to help it evade international sanctions
Elsewhere, amid much speculation over the impact of Donald Trump's re-election victory last week, the Kremlin has denied media reports that he held a phone call with President Vladimir Putin.
The call, which was first reported by the Washington Post on Sunday, is said to have happened on Thursday. Trump is said to have warned the Russian president against escalating the war in Ukraine and mentioned America's extensive military presence in Europe.
Trump's team told the BBC that it would not comment on the president-elect's "private calls".
