Seoul believes North Korea is preparing to launch a ballistic missile, posing a possible new threat to stability on the peninsula ahead of South Korea's presidential election in December.
"It's clear that they're in preparation for a possible launch," a senior foreign ministry official told the Financial Times, while stressing that the South Korean government could not predict when or whether a missile test would take place.
Last week, Japan's Asahi newspaper reported that US intelligence had detected possible launch preparations. In April, a North Korean rocket that Pyongyang said would carry a satellite into orbit failed 90 seconds into flight.
On Monday, DigitalGlobe, a satellite imagery provider, released an image showing what it called a "marked increase in activity" at a launch site near the Chinese border. Similar recent images have convinced Seoul that a launch could be imminent, the South Korean official said.
The official suggested Pyongyang might take such a provocative step in order to seek a stronger negotiating position against South Korea's next president, who will take office in February.
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"At the moment, we don't have much tools or leverage to prevent them from making further provocations," the official said.
A launch would provide an early test for either Park Geun-hye or Moon Jae-in, the two leading contenders in the December 19 election. Both have promised to improve relations with North Korea, with Mr. Moon pledging to reintroduce generous economic support that was withdrawn under the incumbent president, Lee Myung-bak. However, both candidates have also underscored their determination not to tolerate aggression from the North.
Pyongyang is paying close attention to the election race and has criticized the conservative Ms Park. On Monday, the official Rodong Sinmun newspaper accused her and Mr. Lee's New Frontier party of being "wicked confrontation maniacs" and having "wrecked peace and stability on the Korean peninsula".
While Seoul and Washington have condemned previous North Korean missile launches, Pyongyang argues that it should be entitled to launch satellites as a signatory to the Outer Space Treaty, despite a UN Security Council resolution to the contrary.
On Thursday, South Korea will launch a satellite using a ballistic missile, giving an opportunity for Pyongyang to highlight its demand to be afforded the same rights as its neighbor.
Daniel Pinkston, an analyst for the International Crisis Group in Seoul, said the apparent preparations for a launch by the North "shouldn't come as a surprise".
"They've invested a huge amount of financial resources, human resources and political will, and it's very consistent with their state doctrine," he said. "They want to have an [intercontinental ballistic missile] capability and a space launch capability."
Osamu Fujimura, Japan's chief government spokesman, on Tuesday said Japan and North Korea would go ahead with a planned meeting in Beijing in early December, in spite of the reports of missile launch preparations. He added that Japan was keeping a close eye on developments.
Additional reporting by Michiyo Nakamoto in Tokyo