Image caption A tightly guarded border separates North and South Korea

North Korea has demanded that China return 13 defectors who were arrested as they tried to make their way to South Korea.

It said they should be returned "as early as possible" and accused South Korea of "abducting" North Koreans.

The escapees were caught in China's Yunnan province. North Korean refugees tend to make their journey to South Korea by first heading to China.

China often repatriates them to the North, ruling them economic migrants.

China is North Korea's only ally and a major trading partner.

"The DPRK [North Korea] calls upon the countries concerned... to send back its inhabitants as early as possible as required by international law," a statement by North Korea's Committee for the Peaceful Reunification of Korea said on Friday.

It described those who helped the defectors as "agents and human traffic dealers of the South Korean Intelligence Service" and accused them of "abducting inhabitants of the DPRK to forcibly take them to South Korea".

Rights groups say refugees who are repatriated can face punishment and imprisonment.

In June, the UN said it was dismayed over reports that China and Laos forcibly repatriated nine young North Korean refugees.

Escaping North Korea is extremely difficult as all its borders are heavily patrolled.

Most North Korean refugees leave via China and head for nations in South East Asia, from where they can get to South Korea - which provides financial assistance and training.

However, the number of North Korean refugees arriving in South Korea dropped by 40% last year.

Activists say North Korean leader Kim Jong-Un is taking stronger measures to prevent people from trying to leave.

The North and South remain technically at war after the 1950-53 conflict ended in an armistice and not a peace treaty.