(CNN) The daughter of a missing North Korean diplomat who is believed to be seeking asylum in Europe was separated from her parents and returned to Pyongyang, a prominent defector claimed this week.

Thae Yong Ho, who fled his post as North Korea's deputy ambassador to the UK in 2016, said Jo Song Gil, a former envoy to Rome, "wasn't able to leave Italy with his children and North Korea has summoned his children back to North Korea."

Speaking to reporters on Wednesday, Thae said a source in Pyongyang "told me that Jo's daughter was in North Korea under state custody," which he said explained Jo's disappearance since he fled the Rome embassy late last year.

"Today, Jo cannot reveal where he is or engage in public activity because he must fear safety of his daughter," Thae said. He added that while he had previously urged Jo to join him in Seoul, he would no longer advocate this, as "the level of punishment against family left behind of diplomats who defected to South Korea or to the US are very different."

"I couldn't keep up the demand that Jo come to South Korea since I found out that Jo's daughter is in North Korea," he said.