North Korean leader Kim Jong-un has ordered the execution of 15 people this year, including several top officials, South Korean intelligence says.

The agency told a parliamentary meeting on Wednesday that they were executed by firing squad on spying charges.

Those killed include two vice ministers who challenged Kim over his policies and members of an orchestra, according to South Korea.

Kim purged and executed his once-powerful uncle for treachery in 2013.

South Korean politicians were told that one of the officials killed was a forestry minister who had complained about North Korea's forestation plan, Yonhap news agency reported.

Four members of North Korea's Unhasu Orchestra are also believed to have been executed in March.

In 2013, Kim put his uncle, Chang Song-thaek, to death, along with a group of officials who had been close to him.

Chang was seen as a mentor to Kim Jong-un during the leadership transition from his father Kim Jong-il in 2011.