North Korea on Wednesday repatriated two South Koreans in their 50s who were detained last month for illegally entering the communist state, Seoul's Unification Ministry said.

The two -- a 59-year-old man and a 51-year-old woman -- were reported missing while travelling in northeastern China near the border with North Korea.

It remains unclear who they are and why they entered the Stalinist state, but Yonhap news agency said they were believed to be husband and wife.

The two were handed over to the South at Panmunjom, an armistice village sitting on the inter-Korean border, according to the ministry.

They will be questioned to identify the reasons why they entered the North without authorization, an official of the ministry said.

Pyongyang sent home six South Koreans in 2013 and a 52-year-old South Korean man last year after rejecting their requests for defection.

However, it has rejected Seoul's repeated calls to free four other South Koreans, including a New York University student and three missionaries, who were detained after illegally entering the country.

Pyongyang views foreign missionaries with deep suspicion, although it allows access to some who undertake humanitarian work.

However, anyone caught engaging in any unauthorized activities would be subject to immediate arrest.

A number of missionaries – mostly U.S. citizens – have been arrested in North Korea in the past with some of them allowed to return home after interventions by high-profile U.S. figures.

In November, Kenneth Bae -- a Korean-American missionary sentenced to 15 years hard labor in North Korea after being arrested in 2012 and charged with seeking to topple the government -- was released following a secret intervention by U.S. intelligence chief James Clapper.