President Bill Clinton's top defence envoy has sought reassurances from Chinese leaders over Hong Kong's future, saying it is a 'constant' US concern. National Security Adviser Anthony Lake told the South China Morning Post that Beijing had assured him it would guard the development of 'one country, two systems'. 'It's a constant concern . . . and we are watching it very carefully,' Mr Lake said. 'We told the Chinese it is primarily a matter for the Chinese and British governments, but the closer you get to the date naturally the issue becomes more important. 'It is said there will be two systems and I think the short version is that we are wanting to be careful to make sure there is, in fact, two systems.' Mr Lake's comments follow closed-door sessions in Beijing with President Jiang Zemin and premier Li Peng in the first visit by a senior United States official since the Taiwan missile crisis in March. Sources close to the discussions said China was doing its best to be 'upbeat'. 'They made it clear to us autonomy would be preserved,' one US official said. 'They seemed to want us to know they were determined things would go well for Hong Kong.' Mr Lake's visit to China fuelled efforts for reciprocal state visits by President Clinton and President Jiang. Mr Lake left Vietnam for South Korea yesterday after cutting short his trip to China, with US diplomats claiming it was raining too hard for his US Army jet to land in Shanghai. He held strategic talks with South Korean officials on the political and economic situation in North Korea. A South Korean Foreign Ministry spokesman said Mr Lake met Foreign Minister Gong Ro-myung and Yoo Chong-ha, senior presidential secretary for foreign affairs and national security. He then flew to Tokyo.