"The government is deeply concerned at recent developments in North Korea and is watching the situation closely," the South Korean Unification Ministry said on Friday in response to the announcement.

Seoul will prepare fully for "all possibilities in the future" while coordinating closely with its allies, a ministry statement said.

Top South Korean presidential security and government ministers held an unscheduled meeting on Friday to discuss the execution of Jang Song Thaek and its possible consequences, according to the presidential Blue House.

'Reign of terror'

South Korean President Park Geun-hye has accused Kim Jong Un of resorting to a "reign of terror" to cement his leadership.

The United States also expressed concern at the execution.

"If confirmed, this is another example of the extreme brutality of the North Korean regime," US State Department deputy spokeswoman Marie Harf said.

She said Washington, too, was following developments in North Korea closely.

The North Korean state news agency KCNA announced on Friday that Jang had been executed, calling him a "traitor for all ages" and "despicable human scum" who had targeted leader Kim Jong Un with a planned coup.

Earlier in the week, North Korea confirmed that Jang, 67, had been stripped of all posts, and branded him as a drug user and a profligate playboy who had gambled away millions in state funds at foreign casinos.

The description is a complete reversal of the previously propagated image of Jang as a mentor and father figure to Kim Jong Un in the early stages of his rule.

More purges to come?

Although the Kim family has regularly purged those showing the slightest signs of dissent during its six-decade-long rule, Jang's execution, if confirmed, was the highest-level one in the country in decades.

Jang's death has raised fears of a further-reaching purge in the country.

A former senior White House adviser on Asia, Victor Cha, said, "If he (Kim Jong Un) has to go as high as purging and then executing Jang, it tells you that everything's not normal."

Japan's defence minister warned this week that the purge could herald an unheaval comparable to the Cultural Revolution in China, which led to widespread persecution and destruction.

tj/pfd (AFP, AP, dpa)