Taiwan has condemned a new Chinese law giving Beijing the legal right to use force against the island if it moves towards declaring formal independence.
China's leaders fear Taiwan could formally declare independence
Such "serious provocation" gravely affects regional security, said Joseph Wu, chairman of Taiwan's Mainland Affairs Council.
The White House said the adoption of the new law was "unfortunate".
China sees Taiwan as its territory and says it reserves the right to use force if "peaceful reunification" fails.
The new law was passed in the final session of the Chinese parliament's annual National People's Congress by a margin of 2,896 to zero, with two abstentions.
The Congress broke into applause at the passage of the so-called anti-secession law which allows for the use of "non-peaceful and other necessary measures".
Ruled by separate governments since end of Chinese civil war in 1949
China considers the island part of its territory
China has offered a "one country, two systems" solution, like Hong Kong
Most people in Taiwan support status quo
Analysts say the new law is partly designed to limit the options of Taiwanese President Chen Shui-bian, whose Democratic Progressive Party is pro-independence.
But it will add to tensions across the Taiwan Strait, where China has been rapidly building up its military capability.
"The law is tantamount to authorisation of war," warned Taiwanese cabinet spokesman Cho Jung-tai.
"All people in Taiwan are against the legislation, and we believe the world community also opposes it."
President Chen has called for hundreds of thousands of people to join a mass street protest later this month.
US Secretary of State Condoleezza Rice, who will visit Beijing during a tour to Asia this week, said the law was "not necessary".
White House spokesman Scott McClellan told reporters: "We view the adoption of the anti-secession law as unfortunate. It does not serve the purpose of peace and stability in the Taiwan Strait."
The US is Taiwan's closest ally and is worried about being sucked in to any conflict between the island and China.
Japan's Prime Minister, Junichiro Koizumi, also expressed concern.
"I wish both parties would work toward a peaceful solution and I hope that this law will not have negative effects," he said.
The 10-article law calls for the use of "non-peaceful means and other necessary measures to protect China's sovereignty and territorial integrity" if all other efforts fail.
China's Premier, Wen Jiabao, said the law was aimed at improving relations with Taiwan.
"This is a law to strengthen and promote cross-Strait relations, for peaceful reunification, not targeted at the people of Taiwan, nor is it a law of war," he said.
Some analysts have said China's use of the term "non-peaceful means" appears designed to include alternatives to military force, such as blockades or sanctions.
The law's passage comes a day after China's President, Hu Jintao, told China's 2.5 million-strong People's Liberation Army, whose budget has risen rapidly in recent years, to put national defence "above all else".
"We shall step up preparations for possible military struggle and enhance our capabilities to cope with crises, safeguard peace, prevent wars and win the wars if any," the president said.
China's leaders frequently make such statements, which are often directed at Taiwan.