North Korean leader Kim Jong-un ordered the execution of the country's vice premier by shooting earlier this year, sources said Tuesday, in another sign of the leader's reign of terror.

Vice Premier Choe Yong-gon, 63, was executed in May as he had expressed discomfort against the young leader's forestation policy, according to the sources.

The Unification Ministry declined to confirm the speculation, but it is on the lookout for what has become of the vice prime minister due to his absence from the public eye.

"Choe has not been seen publicly for about past eight months since he was last witnessed in December last year at the ceremony to mark the third anniversary of the death of North's former leader Kim Jong-il," the ministry said in a statement. "The government is closely watching to see what might have happened to him."

An official at Seoul's spy agency said he cannot confirm the speculation.

If confirmed, the case adds to a series of high-profile execution of North Korean ranking military and other officials, including the North's former defense chief Hyon Yong-chol.

Hyon is believed to have been executed with an anti-aircraft gun in April for his disloyalty toward Kim as he dozed off during a military event.

In late 2013, Kim Jong-un ordered the execution of Jang Song-thaek, the husband of Kim's aunt and once the country's second most powerful figure, on charges of treason.

The National Intelligence Service earlier said that the North Korean leader has tightened the grip on his power base in a brutal way, including the execution of about 70 senior officials.

Choe, who has served as the vice premier since June 2014, represented the North's delegation in inter-Korean economic talks in Seoul in 2005. (Yonhap)