The United States has warned North Korea against any "provocative acts" following the shock execution of leader Kim Jong-Un's uncle, as the reclusive state campaigned to rally support behind the young supremo.

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 on Friday.

The comments came after South Korea's defence chief Kim Kwan-Jin pledged to increase military vigilance against any potential provocations, saying the stunning purge indicated Kim Jong-Un's firm resolve to tighten his grip on power.

"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 on Friday.

"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."

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.

Meanwhile, the reclusive state on Saturday launched a fresh media blitz in a bid to rationalise Jang's elimination and rally support behind 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.

'Incredibly brutal act'

The inspection trip was apparently aimed at displaying the young Kim's continued stranglehold on power after the shocking elimination of Jang, condemned by Pyongyang as "a traitor for all ages".

Pyongyang also appeared to be cracking down on Jang's loyalists, with Yonhap news agency saying North Korean businessmen in China have been summoned back in large numbers.

The move appeared aimed at cracking the whip on those "classified as having connections" with Jang, who served as a key go-between for relations with China, Yonhap said Saturday citing unnamed sources.

Jang, 67, played a major 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.

Harf, the State Department spokeswoman, denounced his execution as an "incredibly brutal act" that underscores North Korea's "horrific human rights record" but refused to speculate on the reasons behind the purge.

"We're going to increase our discussions with our allies and partners in the region about the internal situation in North Korea," she told reporters.

Washington is in regular talks with Beijing, Pyongyang's sole major ally, and "we're on the same page in terms of urging the North Koreans to come back in line with their international obligations", Harf said.

The regime accused Jang of betraying the trust of both Kim Jong-Un, who is aged around 30, and his father -- saying he had received "deeper trust" from the younger leader in particular.

In a nation long ruled under a pervasive personality cult, Jang was also accused of slighting the young leader -- not applauding him enthusiastically enough at party meetings and blocking the construction of a mosaic in his honour at a tile factory.

Rodong Sinmun sought to stir up hatred against Jang, carrying a lengthy official statement Saturday titled "We know no one but Comrade Kim Jong-Un".

In a separate article titled "Anyone who betrayed the party and the leader's trust has no place on earth to hide", it quoted a construction worker saying: "It is very regrettable that we failed to have a chance to drag Jang like a dog to this dam construction site and put him in the concrete."