Washington also sought to step up talks with its Asian allies, voicing concern over regional stability after Jang Song-Thaek - seen as Kim's political regent and the country's unofficial number two - was executed on Thursday following a special military trial.
"Certainly, it's something we're concerned about, and we would urge the North Koreans not to take provocative acts, not to do so going forward, because it's not in the interest of regional stability," US State Department spokeswoman Marie Harf said yesterday.
"We will heighten readiness against North Korea as (Jang's execution) can lead to provocations against the South," he said in the parliamentary defence meeting yesterday.
"This case can be seen as part of the reign of terror by Kim Jong-Un as he is seeking to consolidate his power with an iron fist."
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He said the Stalinist regime risked misjudging the security situation on the peninsula, voicing concern over rivalry among North Korean military leaders eager to prove their loyalty to the young leader.
Rodong Sinmun, the official daily, splashed on its front page a colour photo of Kim, wearing a great coat with his hands in his pockets, touring a military design institute in his first public activity following the purge.
He was accompanied by Choe Ryong-Hae, a close Kim Jong-Un confidant who holds the military rank of vice marshal, and trailed by other military officers, one of whom was seen jotting down Kim's "field-guidance" instructions in his notebook.
Jang, 67, played a key role in cementing the leadership of the inexperienced Kim when he succeeded his father Kim Jong-Il in 2011, but analysts said his power and influence had become increasingly resented.