South Korea expressed deep concerns Friday after North Korea announced the surprise execution of leader Kim Jong-un's uncle amid fears about its implications for Pyongyang's future, inter-Korean relations and the standoff over the country's nuclear program.

The execution of Jang Song-thaek came as a surprise, as few had expected the young leader would go as far as executing his uncle just four days after the regime held a political bureau meeting of the ruling Workers Party and removed him from all of his posts.

The North's Korean Central News Agency said earlier Friday that Jang was executed Thursday immediately after a special military tribunal sentenced him to death for "such hideous crimes as attempting to overthrow the state," including contemplating a military-backed coup.

"The government has deep concerns about a recent series of developments in North Korea and is watching the situation closely,"

Unification Ministry spokesman Kim Eyi-do said after a meeting of security ministers.

"We will make sure to be prepared for all possibilities in the future," the spokesman said, adding that South Korea will work closely together with allies and other related countries in coping with the situation.

Jang had been considered the No. 2 figure in Pyongyang and held a string of high-level posts, including vice chairman of the powerful National Defense Commission. He was believed to have looked after the young Kim to take over as leader after his father and late leader Kim Jong-il died in 2011.

His execution could have significant implications for Pyongyang's policies.

Jang had been considered a moderate in a regime full of hawkish military generals and an advocated of economic reform. The execution also suggests that a purge campaign against him and his associates is in full swing and more executions could come.

Jang was married to Kim Kyong-hui, the sister of late North Korean leader Kim Jong-il.

In Seoul, presidential national security chief Kim Jang-soo presided over a security ministers' meeting to discuss the execution and its implications for North Korea's future and relations with South Korea and the nuclear standoff.

"There are not many people who expected that an execution would be carried out immediately like this, though we had thought that stern measures would be taken," a government official said on condition of anonymity.

South Korea's military said it has stepped up surveillance on Pyongyang and is maintaining high vigilance against any provocations, though no special movement in the North has been detected yet.

"The Joint Chiefs of Staff has heightened readiness as the North Korean military started winter training earlier this month," a senior military official said. "We have beefed up military readiness and enhanced the system to closely monitor the North's moves."

Sources said the execution may suggest Kim's grip on power may not be as strong as thought.

Since Jang's fall from power was made known by South Korea's main intelligence agency last week, there have also been concerns that the communist nation could resort to military provocations to increase internal unity. (Yonhap News)