Activists say up to 600 North Korean defectors, believed to be mostly women, have not been heard from since their deportation from China.
In October, China carried out the largest repatriation of North Korean detainees in years, after Pyongyang ended its Covid border lockdown.
This put the detainees' lives at risk, human rights groups said at that time.
Viewed as traitors, they can face torture, sexual violence and execution in the isolated state, advocates said.
On Thursday, a Seoul-based human rights group tracking the detainees said they believed up to about 600 North Koreans had "vanished" after being forcibly returned.
Noting that communication in the shut-off regime is always difficult, activists at the Transitional Justice Working Group said they had not been able to establish contact with any of the returned detainees.
A UK-based sister of one of the returned defectors told reporters she feared for her sister's life.
"My sister's only crime was being born in North Korea... All I want is for her to live in safety," said Kim Kyu, the sister of Kim Cheol-ok, one of the few known members of the repatriated group.
Most members' identities are unknown, but the majority are believed to be women.
In October, researchers spotted Chinese-operated buses and vans transporting deportees from detention centres in China's Jilin and Liaoning provinces to five crossing points along the border with North Korea.
The rights group identified the prisoners as believed to have been held at Shenyang Prison and Chanchun Tiebei Prison, and noted over 300 had crossed at the Onsong crossing point on 9 October, while 180 went over at Dandong crossing point around the similar period.
The Chinese government has not made any public acknowledgement of the deportations.
Typically, it does not describe North Koreans detainees as defectors or grant them asylum status, labelling them instead as illegal economic migrants.
Since North Korea's famine in the 1990s, hundreds of thousands of locals have tried to flee the totalitarian regime. More than 70% are women, says the TJWG.
Leader Kim Jong Un has reinforced a crackdown on border security in recent years- coinciding with the country's lockdown during the pandemic.
It re-opened only a few months ago, largely to improve trade with China.
