North Korea has previously claimed the waitresses were abducted to South Korea.
Pyongyang's state-controlled news agency KCNA reported on Tuesday North Korea's Ambassador to the U.N. Ja Song Nam sent the letter to Guterres last Friday, expressing his "disappointment" over the lack of a U.N. response to previous North Korea requests.
"We cannot but express our disappointment at the fact that no action has been taken by the U.N. until now as we have entered 2017 without even a reply to our letters," Ja wrote, according to KCNA. "How to deal with this case will be a touchstone testing the true stand of the U.N. for the promotion and protection of human rights."
The letter also demanded the repatriation of Kim Ryen-hi, a North Korean defector living in the South who has claimed she was kidnapped in June 2011, according to South Korean newspaper Maeil Business.
Kim, who was profiled in The New York Times, has claimed she was smuggled to South Korea. She told The Times in 2015 that she agreed to be smuggled to the South for economic reasons, and to pay for her relatives' medical bills, but ended up losing her North Korea citizenship and being admitted to the South as a defector.
The waitresses who arrived in South Korea last April have been resettled in "various parts of South Korean society," a Seoul government source said in July.
The group escaped from Ryugyong restaurant in Ningbo, China, then entered South Korea.
North Korea is expected to ask for interviews with the group, according to Choi Sung-ryong, a rights activist in the South.
Choi has said a source in North Korea informed him Pyongyang plans to send envoys to the annual meeting of the U.N. Human Rights Council in Geneva.