Thousands of North Korean soldiers are now positioned near Ukraine's border and likely to enter combat in the coming days, the US says.
Russian troops have been training them in artillery, drones and "basic infantry operations, including trench clearing", said US secretary of state Antony Blinken.
He said it strongly indicated they would be used on the front line and would therefore become legitimate targets for Ukraine.
Some 10,000 North Korean troops are in Russia, including up to 8,000 in the Kursk border region, Mr Blinken said.
The troops are wearing Russian uniforms and carrying Russian gear, according to US defence secretary Lloyd Austin.
Read more: Where have North Korea troops been seen in Russia?
"We've not yet seen these troops deploy into combat against Ukrainian forces, but we would expect that to happen in the
coming days," Mr Blinken said on Thursday.
America's top diplomat said the recruitment of troops from North Korea to Russia's "meat grinder" was a "clear sign of weakness".
Mr Blinken made the assessment after he and Mr Austin met their South Korean counterparts in Washington DC.
Foreign minister Cho Tae-yul called for the immediate withdrawal of North Korean soldiers from Russia and condemned it "in the strongest possible terms".
They also all agreed China should do more to rein in North Korea, Mr Blinken said, adding that he'd had a "robust conversation" with Beijing this week.
Mr Austin also announced that - with the US election just days away - America would soon be announcing new security assistance for Ukraine.
The deployment of troops to Russia is down to the close relationship between President Putin and North Korea's Kim Jong Un.
The leaders last met in June when Mr Putin travelled to North Korea for the first time in 24 years.
A mutual defence pact was agreed during their summit, meaning the countries will help each other if they are attacked.
The US says North Korea has also given munitions to Russia as it continues its grinding effort to take more territory in Ukraine's east.
The White House published images earlier this month which it said showed 1,000 containers of equipment being sent to Russia by rail.
There are concerns about what military aid Russia will now provide in exchange.
North Korea test-fired an an intercontinental ballistic missile for the first time in almost a year on Thursday and there is speculation Russia may have provided technological help.
In a statement, the US, Japan and South Korea condemned the launch as a "flagrant violation" of UN resolutions.
"We strongly urge (North Korea) to immediately cease its series of provocative and destabilising actions that threaten peace and security on the Korean Peninsula and beyond," they said.
(c) Sky News 2024: Ukraine war: Thousands of North Korean troops near Ukraine border will enter battl
'People were crying, they were trapped': Spain reels from deadly flash floods
'No one came to rescue us': In this destroyed Spanish town, people are angry
Budget: Hostile market response as chancellor suffers Halloween nightmare
