South Korea's military said on Friday it had detected signs in North Korea of possible preparations for a satellite launch in what would be the nuclear-armed North's effort to place a second military reconnaissance craft in orbit.

South Korea and U.S. intelligence authorities are closely monitoring North Korea's activities, a South Korean military official said.

The activities were spotted at Dongchang-ri, a northwestern area of the country where North Korea's main space flight centre is based, the official said without elaborating.

North Korea launched its first spy satellite last year and claimed leader Kim Jong Un reviewed photos taken of the White House and the Pentagon, among other areas of the world. State media have not released any photos taken by the craft, however.

The launch was met with sanctions from the U.S. and its allies and prompted Seoul to suspend part of a military agreement it signed with Pyongyang in 2018.

Pyongyang scrapped the pact in response and restored previously demolished guard houses near the border.

The reclusive state also vowed to launch three more satellites this year.

Earlier this year, South Korea launched its second homegrown spy satellite on a SpaceX Falcon 9 rocket from the John F. Kennedy Space Center in Florida amid a space race with the North.

Pyongyang has defended its satellite launch as part of its right to self-defence and denounced Washington's reaction as a "double standard" over South Korea's launch of its own.

(Reporting by Hyunsu Yim; Editing by Kim Coghill)