According to the NIS, North Korea repositioned some military aircraft to the east coast, and carried out a surveillance flight in the region sometime after the U.S. aircraft withdrew from the area.
On Monday, North Korean Foreign Minister Ri Yong-ho claimed that the U.S. has declared war on his country, and that USA military planes will be shot down should they approach North Korea airspace.
"If the situation spirals out of control, in South Korea and North Korea, and Japan, Russia, and China, hundreds of thousands of innocent people will suffer". The latter flew bombers over waters east of North Korea on Saturday, and Pyongyang said afterwards that it had every right to shoot down those planes.
He said a Twitter message by Trump on Saturday in which the president warned that the minister and North Korean leader Kim Jong Un "won't be around much longer" if they acted on their threats amounted to a declaration of war.
North Korean Foreign Minister Ri Yong-ho also staged the latest episode of the verbal war in NY on Monday, after saying that the US President's statement to the United Nations, in which he threatened to "totally destroy North Korea", was a declaration of war against his country.
In his first speech at the UN General Assembly on September 19, Trump threatened to "totally destroy" North Korea if necessary.
In addition to the nuclear test, North Korea has launched dozens of missiles since Kim came to power in 2011.
North Korea has not commented on the report. Pyongyang says it needs those programs as a deterrent against hostility by the U.S. and its regional allies, including South Korea and Japan.
The National Assembly's Intelligence Committee Chairman Lee Cheol-woo told reporters that the NIS made the confirmation during a meeting with the parliamentary committee on Tuesday.
'We have not declared war against North Korea and frankly the suggestion of that is absurd, ' said White House press secretary Sarah Huckabee Sanders.