China has expressed strong concerns over long-time ally North Korea's plans to go ahead with a controversial satellite mission and rocket launch this week, even as South Korean officials warned on Monday that their reclusive neighbour may also be preparing for a third nuclear test.

With tensions in the region already running high over the North's plans to launch a satellite — a move that its neighbours say violates United Nations Security Council (UNSC) resolutions — South Korean intelligence officials said on Monday that satellite images showed plans for a third nuclear test.

South Korea's Yonhap news agency quoted officials as saying “clandestine preparations” were being made at the Punggye-ri test site, which saw tests in 2006 and 2009 that triggered regional tensions.

On both occasions, officials pointed out, nuclear tests followed rocket launches. Images showed tunnels being dug and construction “in its final stage”.

The North has announced it will launch a satellite, carried by a long-range rocket, to mark the 100th birth anniversary of the founder of the Democratic People's Republic of Korea (DPRK), Kim Il-sung.

The moves are also seen as aimed at consolidating the position of the young and inexperienced leader, Kim Jong-un, who took over in December following the death of his father, and Kim Il-sung's son, Kim Jong-il.

South Korean officials said the North had already set up the three-stage boosters and was moving the payload to the launch pad, Yonhap reported.

Chinese Foreign Minister Yang Jiechi said China was concerned by the moves and urged “restraint” from all parties, following a trilateral Foreign Ministers' meeting among China, Japan and South Korea in the Zhejiang province.

Both Japanese and South Korean leaders have warned they will shoot down the rocket if any of its parts head for their territory, a move the North said would be seen as an act of war.

The North's launch plans are part of a pledge to make the nation “strong and prosperous” by 2012, the 100th birth anniversary of its founder. With the North far from economic prosperity and reeling from dire food shortages — the U.S. has also suspended an agreement for food aid because of the launch — its leaders were focusing on showing the nation's military strength, analysts said.

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