SEOUL -- Monday's collision between a North Korean fishing boat and a Japanese patrol ship in the Sea of Japan is the latest in a string of international incidents involving North Korean fishers forced into riskier waters as Chinese competitors crowd them out of their usual grounds.

The incident took place around 9:10 a.m. near the Yamato Bank, a rich fishing ground within Japan's exclusive economic zone. After being warned to leave the area, a large North Korean boat collided with a Japanese Fisheries Agency patrol vessel and sank about 20 minutes later. Dozens of crew members were rescued.

The upswing in North Korean fishing in foreign waters owes to the sanctions-battered North's sale of fishing rights along its coast to China to secure desperately needed foreign currency. The South Korean government estimated in 2016 that Pyongyang earned tens of millions of dollars from the deal.

But with Chinese ships now operating along the coast, North Korean fishers are forced to risk venturing farther out, often in foreign waters.

Since September, Russia has apprehended more than 800 North Koreans fishing within its exclusive economic zone. A ship and motorboat seizure late that month turned up about 30,000 illegally caught squid, according to Russian media. In another incident, Russian border guards were injured as crew members of a detained vessel resisted arrest.

South Korea has seen a surge since this summer in North Korean fishing operations near the Northern Limit Line, the disputed maritime boundary between the two Koreas. The number of illegal crossings swelled from 51 in all of 2018 to more than 400 so far this year, data from the South Korean military shows.

The shift to offshore and distant-water fishing has been actively encouraged by North Korean leader Kim Jong Un. The Korean Central News Agency reported late last year on inspections of military-run offshore fishery stations by Kim, who said the fish-filled cold storage facilities looked "like a treasure mountain and gold bars."

It has been said that Pyongyang looks to smuggle catches into China to meet seafood demand there.

The North Korean government has begun taking a more aggressive stance on fishing-related incidents. After a Japan Coast Guard vessel came into contact with a North Korean fishing boat this August, the Ministry of Foreign Affairs in Pyongyang responded to a protest later lodged by Tokyo by asserting that North Korea had chased the Japan Coast Guard out of its own waters.

The ministry called this "a just and righteous exercise of our sovereign rights" and said it "sternly alerted" Tokyo of the need to prevent "any possible reoccurrence of such acts of intruding into our zone and obstructing the activities of our fishing vessels."