"If anyone dares to separate Taiwan from China, a Chinese military will not hesitate for a second," Li said in a speech at the Shangri-La Dialogue in Singapore, a conference organised by the International Institute for Strategic Studies.
"We will fear no opponents and resolutely safeguard national sovereignty and territory integrity regardless of any cost."
Taiwan has been independent since 1949, but China considers the self-ruled democracy to be Chinese territory.
Li criticised US arms sales and military training to Taiwan, as well as the upgrading of Washington-Taipei relations.
"Taiwan is an internal affair of China," the minister said on Sunday at the security summit.
"Taiwan is China's Taiwan and how to resolve the Taiwan question is a matter for the Chinese to decide. Any act to obscure hollowed the one China principle is both absurd and dangerous.
"China's reunification is an overriding historical trend and an unstoppable course.
"We will strive for the prospect of peaceful reunification with utmost sincerity and greatest efforts. But we make no promise to renounce the use of force. If anyone dares to separate Taiwan from China, a Chinese military will not hesitate for a second."
Li described Beijing's relations with Washington at a "record low" since relations began in 1979.
When asked if China was fundamentally open to talks between the two powers, Li said only if they are "based on mutual respect."
The general had turned down the US request for a meeting with US Secretary of Defence Lloyd Austin on the sidelines of the conference.
The Shangri-La Dialogue at the level of defence ministers is seen as the most significant security forum in the region. Forty countries have sent representatives.
Relations between China and the US have hit a low following the downing of an apparent Chinese espionage balloon in February and the cancellation of a visit by US Secretary of State Antony Blinken to China.
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