VIENTIANE, Laos — For more than two years, Lee Jin-hui, 20, was never allowed to leave a three-room apartment in northeast China. Seven days a week, she had to sit at a computer from noon to 5 a.m., performing sex acts before a webcam for male clients, mostly from South Korea.

In the apartment, Ms. Lee and other North Korean women each had to earn about $820 a week for the Chinese pimp who bought them from human traffickers. When they failed, they were slapped, kicked and denied food.

“We had to work even when we were sick,” Ms. Lee said. “I wanted to get out so badly, but all I could do was peek out the window.”

Each year, human smugglers take thousands of women seeking to flee North Korea, promising them jobs in China, according to human rights groups and trafficking survivors. But once in China, many of the women are sold to unmarried men in rural towns or to pimps for exploitation in brothels and cybersex dens.