China claims democratically governed Taiwan as its own territory and in the past two years has stepped up military and diplomatic pressure to assert its sovereignty claim, fuelling anger in Taipei and concern in Washington.
China was willing to try its utmost to seek peaceful reunification with Taiwan but would act if any red lines on independence were crossed, Ma Xiaoguang, spokesman of the Taiwan Affairs Office, told a media briefing.
"If separatist forces in Taiwan seeking independence provoke, exert force or even break through any red line, we will have to take drastic measures," Ma said.
Taiwan has emerged as a key factor in strained relations between China and the United States, the island's most important international backer and arms supplier despite the absence of formal diplomatic ties.
China regularly describes the island as the most sensitive issue in its ties with the United States.
Ma said provocation by pro-independence forces and "external intervention" could grow "sharper and more intense" in coming months.
"Next year, the Taiwan Strait situation will become more complex and severe," he said.
Beijing has sent repeated air missions over the Taiwan Strait in recent months to pressure Taiwan. It has said it will not give in to threats.
While the United States recognises only one China, it is required by law to provide Taiwan with the means to defend itself and has long followed a policy of "strategic ambiguity" on whether it would intervene militarily to protect Taiwan in the event of a Chinese attack.
The defeated Republic of China government fled to Taiwan in 1949 after losing a civil war with the Communists, who established the People's Republic of China.