The North Korean ship, believed to be a fishing vessel, capsized after crashing with a Japanese patrol boat Okuni some 350 kilometres (217 miles) northwest of Noto peninsula in Ishikawa prefecture in central Japan.
A North Korean boat collided with a Japanese fisheries agency patrol vessel in the Sea of Japan Monday, sending around 20 North Koreans overboard, Japan's coastguard said.
"We rescued some 10 people but their condition has not yet been confirmed," a coastguard spokesman told AFP.
Satoshi Kuwahara, a Japanese fisheries agency official, told reporters the patrol boat was warning the North Korea vessel to leave the area, which falls under Japan's exclusive economic zone.
When foreign vessels illegally operate in Japan's waters, Japanese patrol boats often send an audio message, display a warning sign on an electric bulletin board, or spray water at them, Kuwahara said.
The North Korean ship, believed to be a fishing vessel, capsized after the crash with Japanese patrol boat Okuni, the spokesman added.
Local media said no crew members on the Japanese patrol vessel were injured.
The collision occurred in waters some 350 kilometres (217 miles) northwest of Noto peninsula in Ishikawa prefecture in central Japan, said Kazuma Nohara, another coastguard spokesman.
"We've been strengthening patrols around the water there in cooperation with the fisheries agency," in recent years following reports that many North Korean fishing boats were poaching fish and squids, he said.
Experts say some fishermen from the North are travelling far out to sea in order to satisfy government mandates for bigger catches.
But their old and poorly equipped vessels are prone to mechanical faults and other problems, including running out of fuel, and there are few ways for them to call for rescue.
A record 225 suspected North Korean fishing vessels washed up on Japan's coast last year, according to the coastguard.
Boats have also washed up on Japanese shores with the crew on board dead – referred to as "ghost ships" by local media.
In 2018, 10 North Koreans rescued from a tiny wooden boat drifting off northern Japan were deported back to their country.