South Korea's presidential office denounced on Wednesday the repatriation of two North Korean fishermen in 2019 as a potential "crime against humanity" by the previous government.

The comments came after Seoul's Unification Ministry, which handles relations with the North, released photographs of the two men, who were suspected of killing 16 shipmates, being forcibly dragged across the border between the neighbours.

"If they were forcibly repatriated to the North even when they expressed their will to defect, it's a crime against humanity that violated both international law and the constitution," Kang In-sun, spokeswoman for President Yoon Suk-yeol, told reporters.

She vowed that Yoon's administration would uncover the truth behind the decision, as South Korean prosecutors have re-opened an investigation into the case.

At the time of the deportation, the government of then president Moon Jae-in called the fishermen "dangerous criminals" who had killed colleagues in a fight over an abusive captain on their ship before crossing the sea border.

Moon has not commented on the renewed allegations, but his Democratic Party defended the move as being legal and in the national interest.

"Moon's government put the life and safety of the people first, which was the basic responsibility of the country at the time," party members said in a statement on Wednesday.

Human rights activists have condemned the repatriation, and a U.N. investigator said South Korea had probably violated the men's rights.

While their fate has not been confirmed, there was an expectation their rights would be violated when they were turned over to North Korean authorities, Tomas Ojea Quintana, the U.N. rights official, said during a visit to Seoul last month.

This meant Seoul had an obligation to deal with them in the South Korean justice system rather than immediately repatriate them, he added.

"The two men's desperate resistance to being forced back that is so apparent in those photos shows that they understood they were fighting for their lives," Phil Robertson, an official of New York-based Human Rights Watch, said in a statement.

"Moon Jae-in and his officials knew that too, and yet still they sent them back."

Robertson accused Moon of being "so desperate to please North Korean supreme leader Kim Jong Un" that he "shamefully disregarded basic principles of human rights and humanity".