North Korea calls defectors "human scum" or citizens who have been kidnapped or coaxed into leaving. Its government-run website recently called the growing campaign to stop the repatriation a "grave provocation" that tarnishes Pyongyang's reputation.
Seoul has engaged in years of behind-the-scenes diplomacy with Beijing on North Korean migrants, but the government's stance has toughened amid the protests.
South Korean President Lee Myung-bak and other senior officials have publicly pressed China in recent weeks. South Korean Foreign Minister Kim Sung-hwan raised the issue in a recent meeting with his Chinese counterpart Yang Jiechi, but failed to win a pledge to stop the repatriation.
Now Kim is expected to seek help from U.N. Secretary-General Ban Ki-moon and U.S. Secretary of State Hillary Clinton during a visit to the United States this week.
More than 21,000 North Korean defectors have arrived in South Korea since the end of the 1950-53 Korean War, according to Seoul's Unification Ministry. Most crossed the border between China and North Korea.
The 2010 U.S. State Department Human Rights Report says non-governmental organizations' estimates of North Koreans living in China are anywhere from thousands to hundreds of thousands. Activist groups often put the number above 100,000.
North Koreans in China face "constant fear of forced repatriation by authorities" and are vulnerable to human traffickers, according to the State Department report, which was released last year.
China fears that easing its stance could prompt more people to leave the North and threaten China's border security, according to Kim Sung-joo, a political science professor at Seoul's Sungkyunkwan University.
The friction between Seoul and Beijing comes during diplomatic efforts to settle the North Korean nuclear standoff. China hosts the currently stalled six-nation disarmament talks and has acted as mediator since the North walked away from negotiations in April 2009. A food-aid-for-nuclear-concessions deal between the U.S. and North Korea, announced last week, has raised hopes that the six-nation talks might resume.
An official at Seoul's Foreign Ministry, speaking on condition of anonymity because it is department policy, played down the possibility that the disagreement over the North Koreans could affect disarmament efforts, but said Chinese diplomats could be annoyed by Seoul's stepped-up pressure.
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