China has condemned international organizations, which help smuggle North Koreans in and out of China. Authorities have arrested scores of North Korean refugees since last week.
Chinese Foreign Ministry spokeswoman Zhang Qiyue said Beijing is launching an investigation into organizations that help smuggle North Koreans into China on their way to asylum in third countries.
She told reporters in Beijing Tuesday that the Chinese government will continue to crack down on such illegal activities.
She said these actions disturbed public security in China and warned that security officials will do what is necessary.
Chinese police posing as smugglers last week arrested dozens of North Korean refugees who were about to leave China by sea to seek asylum in South Korea or Japan.
In the latest incident, 48 North Koreans were detained along with three aid workers after they arrived in the port city of Yantai, in Shandong province, south of Beijing.
The international aid group Doctors Without Borders said more than 3,000 North Koreans have been arrested since December as China stepped up a crackdown on illegal immigrants. They said they will face a grim fate back in North Korea.
Some 300,000 North Koreans are believed to be hiding in northeastern China, escaping famine and repression at home.
China has a treaty with Pyongyang requiring it to repatriate any illegal North Koreans whom Beijing considers economic migrants not asylum seekers.
Aid workers are now saying China has destroyed 80 percent of the refugee support networks on the border of China and North Korea.
Activists held a news conference in Tokyo Tuesday to blast China for its actions. Norbert Vollertsen is a medical doctor and prominent campaigner for North Korean refugees.
"China is our main target. They are denying human principles and rights. They want to be a member of the international community, look what they are doing," Mr. Vollertsen said.
South Korean Pastor and activist Chun Ki-won, who had been detained in China previously, said Chinese authorities are setting traps for desperate North Koreans.
Pastor Chun said police are conducting a house-to-house campaign along the border, forcing Chinese residents to sign pledges not help North Koreans or face a stiff fine. He said rewards are also being offered for information on any illegal arrivals.
Aid workers believe the campaign is timed to take advantage of the nuclear crisis in northeast Asia and the possible war against Iraq, which is diverting attention away from the plight of North Korean asylum seekers.