Tomas Ojea Quintana, the UN's special rapporteur on human rights in North Korea, said last week he had shared his concerns with Chinese officials in recent months.
But foreign ministry spokesman Lu Kang said on Monday that "the persons who illegally trespassed into China are not refugees" and would be handled according to "international law and humanitarian need".
"We hope relevant people can abide by principles in the UN charter to respect normal law enforcement on the Chinese side," Lu told a regular press briefing.
The envoy said he had reminded Chinese authorities of their obligations under the 1951 refugee convention as well as relevant parts of the UN system.
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"I urge them again to address this problem by giving special protection to DPRK (North Korea) citizens who transit through China's territory," he said in Seoul.
If forcibly returned, North Korean defectors are at risk of persecution, arbitrary detention, torture or other ill- treatment, enforced disappearance and execution, according to Human Rights Watch.
Human Rights Watch said in June, citing activists and family members, that at least 51 North Koreans had been detained in China since July last year, including a baby born in detention, four children and three elderly women in frail health.
Based on their information, the rights watchdog said it believed that at least 13 of the North Koreans had already been forcibly returned, while the others remained in China for now.
The number of North Koreans escaping to the South declined sharply in the first half of this year as Pyongyang strengthened controls on its border with China, Seoul officials said this month.
In the six months to June, 593 Northerners entered South Korea, down 20.8 percent from the same period in 2016, statistics compiled by Seoul's Unification Ministry showed.