SOUTH Koreaon Wednesday launched a civilian space rocket and satellite as signs mount that North Korea, which sent a satellite into orbit last month, is preparing to test a nuclear device.

South Korea has spent about $500m on the project, chasing a toehold in the space services market estimated to provide a 5.5-trillion won ($5bn) boost to the economy. North Korea is threatening to conduct its third nuclear test in response to tighter United Nations (UN) sanctions imposed after last month’s launch.

The 33m KSLV-I Naro rocket was launched from a site near Goheung, 330km south of Seoul, in the afternoon, Education, Science and Technology Minister Lee Ju-ho said. Naro, which is built with domestic and Russian technology and carries a 100kg research satellite, has failed to launch twice since 2009.

"South Korea’s Naro rocket launch won’t be a threat to the North but will give it an opportunity to criticise the UN Security Council for what it sees as double standards, pointing out that the South is permitted to launch satellites when the North is banned," said Yang Moo-jin, a professor at the University of North Korean Studies in Seoul.

North Korean leader Kim Jong-un has maintained the military-first policy he inherited from his father, Kim Jong-il, more than a year ago. Last Thursday, his regime threatened to detonate an atomic device after the Security Council passed the resolution, and two days later he vowed "high-profile" retaliation against the US and its allies.

Satellite images show a "continued state of readiness" at the Punggye-ri nuclear site, according to the 38 North website of the US-Korea Institute at Johns Hopkins School of Advanced International Studies.

South Korean defence ministry spokesman Kim Min-seok went further on Tuesday, saying North Korea was ready to conduct a nuclear test "at any time". He said North Korea may also seek other ways to provoke the international community.

Mr Kim’s regime has enough weapons-grade plutonium for as many as eight basic nuclear weapons, according to estimates by Stanford University nuclear scientist Siegfried Hecker, who visited North Korea’s atomic uranium-enrichment facilities in 2010.

A successful Naro launch would bring South Korea economic benefits of as much as 5.5-trillion won by 2020 from higher value-added exports and a share of the global space-services market, the Hyundai Research Institute said. The satellite will be used for research including measuring space radiation.

The Korea Aerospace Research Institute is leading the project, in collaboration with about 150 South Korean firms, including Korean Air Lines, Hyundai Heavy Industries and Doosan Infracore, the agency said on its website.