South Korea's main opposition party says that it is sending a fact-finding mission to China to look into reports that Beijing deported nine North Korean defectors to the North.

On Thursday, Seoul's Foreign Ministry offered a public apology after acknowledging it failed to protect the North Koreans who sought asylum in the South Korean Consulate in Shenyang, northeastern China, late last year.

The North Koreans, all family members of South Korean prisoners of war from the Korean War, were arrested by Chinese security officials while they were staying in a house provided by the South Korean Consulate.

According to a source, one of the nine, who was the wife of a South Korean prisoner of war, allegedly froze to death one month ago while she was being questioned by North Korean security authorities after the defectors were deported back to the North.

But it is unclear whether her death was caused by harsh punishment during questioning or actually a result of disease, the source said.