North Korea warned on Monday it would continue to exercise its sovereign rights, including through satellite launches, while its troops were reported to be restoring some demolished guard posts on the border with South Korea.

North Korea claims it launched first spy satellite.

North Korea's foreign ministry said the launch of a reconnaissance satellite last week was prompted by the need to monitor the United States and its allies, state media KCNA reported.

"It is a legal and just way to exercise its right to defend itself and thoroughly respond to and precisely monitor the serious military action by the U.S. and its followers," the KCNA report said.

Nuclear-armed North Korea launched the satellite on Tuesday, saying it successfully entered orbit and was transmitting photographs, but South Korean defence officials and analysts said its capabilities have not been independently verified.

The launch prompted South Korea to suspend a key clause in a 2018 inter-Korean military agreement and resume aerial surveillance near the border.

North Korea in turn declared it was no longer bound by the agreement and would deploy weapons on the border with the South.

South Korea's defence ministry said North Korean soldiers had been observed bringing back heavy weapons into the Demilitarized Zone (DMZ) border and setting up guard posts that the two countries demolished under the agreement.

South Korea estimates the North had about 160 guard posts along the DMZ and the South had 60. Each side demolished 11 of them after the military deal signed in 2018 aimed at de-escalating tension and prevent accident military clashes.

Armed North Korean soldiers had been spotted restoring damaged guard posts in several locations since Friday, South Korea's defence ministry said, citing photographs from cameras in the DMZ.

They were also setting up what appeared to be a recoilless rifle - a portable anti-vehicle weapon or light artillery piece - at a fortification, it said, also citing a photograph.

North Korean leader Kim Jong Un again visited the control centre of the space agency in Pyongyang on Monday morning and viewed fresh satellite photos of the United States' Anderson Air Force base in Guam and other places including Rome, KCNA reported.

South Korean President Yoon Suk Yeol was briefed on the latest North Korean activities and ordered military readiness, his office said.

The United States had called an unscheduled meeting of the U.N. Security Council on Monday to discuss the North's satellite launch.

On Nov. 22, nine members of the Security Council joined the United States in a statement condemning the North's satellite launch for using ballistic missile technology, calling it a violation of multiple Security Council resolutions.

North Korea's foreign ministry said the statement only showed how dysfunctional the Security Council had become, with some member states blindly following the United States in issuing meaningless statements.

Two of the veto-wielding permanent members, China and Russia, have refused to join in any new Security Council sanctions against Pyongyang despite its continued testing of increasingly powerful ballistic missiles.