Seoul: North Korea said on Monday it had told the United States it will cut the only channel of communication between them, at the United Nations in New York, after Washington blacklisted leader Kim Jong Un last week for human rights abuses.
All matters related to the United States, including the handling of US citizens detained by Pyongyang, will be conducted under its "wartime law," its official KCNA news agency said.
The move was the latest escalation of tension with the isolated nuclear-armed country, which earlier on Monday threatened a "physical response" after the United States and South Korea said they would deploy the THAAD missile defence system in South Korea.
North Korea said last week it was planning its toughest response to what it deemed a US "declaration of war" after Washington announced sanctions on Kim Jong Un.
A US-based North Korea monitoring project, 38 North, said on Monday that satellite images from July 7, a day after the sanctions announcement, showed a high level of activity at North Korea`s nuclear test site, but it is unclear whether this was for maintenance or preparation for a fifth nuclear test.
"As the United States will not accept our demand for the immediate withdrawal of the sanctions measure, we will be taking corresponding actions in steps," KCNA said on Monday.
"As the first step, we have notified that the New York contact channel that has been the only existing channel of contact will be completely severed," it said.
"The Republic will handle all matters arising between us and the United States from now on under our wartime laws, and the matters of Americans detained are no exception to this."
US State Department spokesman John Kirby declined to comment specifically on the North Korean statement but said such rhetoric "obviously is not doing anything to ease tensions."
Two Americans are currently known to be detained in North Korea. Otto Warmbier, a University of Virginia student, was sentenced in March to 15 years of hard labour for trying to steal an item with a propaganda slogan and Korean-American Kim Dong Chul is serving a 10-year sentence for espionage, according to North Korean state media.
Kirby repeated a call for North Korea to release the Americans from "improper and unjust detention" and stressed the need for it to adhere to its Vienna Convention commitment to allow consular access.
North Korea has previously indicated that wartime laws would mean detainees will not be released on humanitarian grounds.
This could delay release of the Americans, giving North Korea one of its last bits of leverage in negotiations with the United States, said T. Kumar, Amnesty International USA`s international advocacy director.
"The tension is at one of the highest levels now, and one of the areas they have control over is with the detainees," Kumar said. "They will use them as bargaining chips to get some advantages."
Kumar said he did not think the prisoners would be affected in other significant ways.
North Korea and the United States remain technically at war because the 1950-53 Korean War, in which Washington sided with South Korea, ended only with a truce.
The so-called New York channel, via North Korea’s mission to the United Nations, has been an intermittent point of contact between North Korea and the United States, which do not have diplomatic relations.