China has launched a crackdown on illegal immigration in its northeast region bordering North Korea, state press said Friday, in a campaign apparently targeting both North and South Koreans.

The crackdown will run in the Yanbian Korean Autonomous Prefecture, where North Koreans have long fled to escape political oppression and famine in their homeland, from May 15 to October 15, the China Daily reported.

"Foreigners who illegally enter, work and overstay are hidden troubles and they might pose potential threats to social stability," the paper quoted Yanbian police official Li Yongxue as saying.

The Yanbian government refused to comment on the crackdown, which coincides with a campaign against foreigners living illegally in Beijing, when contacted by AFP. Phones at the police station went unanswered.

Tens of thousands of North Koreans have fled poverty and repression in their homeland, almost all of them across the border to China, which regards the fugitives as economic migrants and arrests and repatriates them.

South Korean non-government groups that have set up operations in China -- often illicitly -- to help North Korean defectors said the crackdown would make it even more difficult to get them to safety.

"We were lucky and managed to get some defectors to Vietnam last week, but now it's extremely difficult for them to move in China because of a tougher crackdown," said Chun Ki-Won, a pastor at the Seoul-based Christian organisation Durihana Mission.

"It will get more difficult than before," Chun told AFP.

According to South Korea media some 10,000 to 15,000 North Korean defectors and other illegal migrants are believed to be living in Yanbian, while some 10,000 South Koreans live there as well.

China's official Legal Daily said about 200,000 foreigners visited Yanbian in 2011, but only 5,600 had permanent residence visas.

South Korea and international rights groups have urged Beijing to change its policy of forcibly returning defectors to North Korea, saying returnees can face harsh punishment.

Last week, China reportedly allowed six North Korean refugees to leave for South Korea after they spent months holed up in Seoul's consular offices in China.

China's decision to let the defectors leave came before a May 14 meeting between South Korean President Lee Myung-Bak and Chinese President Hu Jintao, the Korea JoongAng Daily said.