President George W Bush has said America will come to the defence of Taiwan in the event of a Chinese invasion.
His comments come after China delivered a strong condemnation of the White House's decision to sell Taiwan a substantial number of submarines, aircraft and destroyers.
The BBC's Washington correspondent, Richard Lister, said the United States has rarely, if ever, stated its position on Taiwan so explicitly.
Nothing has really changed in policy... this is what other presidents have said and I will continue to say so
In an interview with an American news programme, Mr Bush insisted that this did not indicate a change in policy, only a clarification of an existing position.
He told ABC's Good Morning America on Wednesday that the use of US military force "is certainly an option" if China invades the island.
But in a later interview with CNN, Mr Bush said his comments should not be seen as an endorsement of independence for Taiwan.
"I think it is important for people to also note that my administration strongly supports the One China policy, and we expect any dispute to be resolved peacefully," Mr Bush told CNN.
"Nothing has really changed in policy as far as I am concerned, this is what other presidents have said and I will continue to say so."
The BBC's Tom Carver in Washington says the president's comments reflect not so much a change in policy, but a change in emphasis.
President Bush: Not worried
He says that while Mr Bush wants to spell out to China that the US would come to Taiwan's aid, his warning has been tempered by promises of greater pragmatism.
The president also pledged to abandon the annual review of arms sales to Taiwan.
China considers Taiwan a renegade province which must be reunited with the mainland, by force if necessary.
Security officials have always privately admitted that if China did blockade or invade Taiwan, the Pentagon would have no choice but to provide direct military assistance.
China has warned that America's decision to sell advanced weaponry to Taiwan would cause devastating damage to Sino-US relations.
Beijing said it reserved the right to take further action.
The deal offers a package of ships, anti-submarine aircraft and submarines, but not the sophisticated naval radar system Aegis, which Taiwan had requested.
The US says Taiwan needs to beef up its defences in response to a growing threat from China. Beijing rejects these claims, and accuses the US of violating its sovereignty.
Sino-US relations are already strained following the collision between an American spy plane and a Chinese fighter aircraft on 1 April.
However, President Bush has said he does not expect either the sale, or the spy plane stand-off to affect his state visit to China in the autumn.