China's foreign minister has urged the US, Taiwan's biggest arms provider, to take "concrete measures" against any Taiwanese bids for independence.
Mr Li said Taiwan was a key sticking point in ties with the US
Li Zhaoxing, speaking on the sidelines of China's annual parliament session, said Taiwan was the most important issue facing China-US relations.
Chinese President Hu Jintao is due to visit Washington next month.
Beijing has been angered by Taiwan's recent decision to scrap a council on reunification with the mainland.
"We hope that the US side... recognises the dangerous nature of Taiwan independence secessionist forces and takes concrete measures to oppose Taiwan independence forces," Mr Li told journalists.
China and Taiwan have been governed separately since a civil war ended in 1949, but China still sees Taiwan as its territory and has threatened to use force if the island moves towards declaring independence.
China believes that Taiwan's President Chen Shui-bian, by measures such as scrapping the National Unification Council, is edging Taiwan towards formal independence.
The US, Taiwan's closest ally in the face of China's threat, has made clear that it supports the status quo.
The US called last week for Taipei to "unambiguously affirm" that President Chen Shui-bian's announcement that the National Unification Council and its guidelines would "cease to function" did not mean they had been completely abolished.
Taiwan on Tuesday complained that the mainland's military threat was increasing. China on Sunday announced defence spending would increase by 14% - the latest in a series of big annual rises which have prompted concern from its neighbours.
Mr Li defended the rise, pointing out that China's military spending, in per capita terms, was just 1/77 of that of the US. He added that China would only use weapons in defence.
"China's national defence policy is transparent, it is completely defensive in nature," he told a news conference.
Japan also raised concerns about China's military on Tuesday, saying in a report that information on this issue remained "opaque".
"We believe that it is important for China to improve transparency regarding its military in order to clear fears held by its neighbouring countries," the report said.
Relations between China and Japan remain tense. Mr Li repeated a call on Tuesday for Japanese leaders to stop visiting a controversial war shrine.
Japanese Prime Minister Junichiro Koizumi has visited the Yasukuni Shrine five times since taking office in 2001.