"It is not true," Yoon Il-Young, spokesman for South Korea's defence ministry, told AFP. Seoul's foreign ministry also strenuously denied the report which appeared in the New York Times. "I do not really understand why this inaccurate report has come out," a foreign ministry official complained.

The newspaper said at the weekend that South Korea was "trying to develop longer-range ballistic missiles while keeping some of the programme's key aims secret from Washington." It quoted Pentagon analysts as saying that US spy satellite photographs revealed last year that South Korea had built a rocket motor test station to secretly develop longer-range missiles.

In April, South Korea conducted a short flight test of a new missile that appeared to violate its agreements with the United States, the Times quoted American officials assaying.

South Korea's missile range is set at 180 kilometres (108 miles) by a 1970s agreement with Washington aimed at limiting missile proliferation on the Korean peninsula. But Seoul seeks to increase it to 300 kilometres. The US report also came ahead of US-South Korean missile non-proliferation talks on the extension of South Korea's missile range in Seoul later this week.

Robert Einhorn, US assistant secretary Of state for non-proliferation, was due to visit Seoul from November 18-20 for talks with Song Min-Sun, head of the American Affairs Bureau of the South's foreign ministry. The round will be the second of its kind this year: in September Einhorn and Song met in Washington to discuss the same issue. "As to the missile test fire in April, Seoul already clarified to Washington that it was not violating the missile-range accord," another foreign ministry official said.