The official, who spoke to the Reuters news agency on condition of anonymity, declined to elaborate on the positions of the two ships and could identify only the USS Kitty Hawk, a carrier based in Japan, by name.
The Kitty Hawk has approached Taiwan before during previous elections and observers say the move is usually intended to discourage any military action from mainland China, about 160km northwest of the island.
The defence official said the carriers were not near enough to the island to provoke China but would be able to "respond if there's a provocative situation".
US navy officials said the Kitty Hawk left its base in Japan en route to Hawaii on Tuesday and would continue on to the US mainland later for decommissioning.
China has repeatedly threatened to invade self-governing Taiwan if the island, which Beijing regards as its own, makes moves to formalise its de facto independence.
Taiwan split from mainland China at the end of the Chinese civil war in 1949
Beijing regards Taiwan as part of its territory, and says it will use force if it ever declares independence
Taiwan has been a multi-party democracy since 1996
Taiwan's defence ministry says China now has nearly 1,000 missiles aimed at the island
The US is the major arms supplier to Taiwan and has warned China that any attack on the island would be viewed with "grave concern"
A Chinese invasion of Taiwan could raise the prospect of a war between China and the US because American law compels Washington to come to the aid of Taiwan if its security is threatened.
On Tuesday, Wen Jiabao, the Chinese premier, urged Taiwanese voters to reject a referendum being held alongside the presidential vote on whether the island should join the UN.
"That would deal a serious blow to cross-strait relations. That would harm the fundamental interests of the people on both sides, cause tensions in the Taiwan Strait and threaten peace in the Taiwan Strait and the Asia-Pacific region at large," he said.
The referendum happens alongside presidential elections that pit Ma Ying-jeou of the Kuomintang party against Frank Hsieh of the ruling Democratic Progressive party.
Both candidates have advocated a more moderate tone in relations with China than has been the case under the current president, Chen Shui-bian.
The US has said it too is opposed to the referendum, which it views as unnecessarily provocative.
"We believe it is unnecessary and unhelpful and will not have an effect on Taiwan's ability to join the UN or other organisations requiring statehood and has the potential to raise tensions in the Taiwan Strait," Tom Casey, the state department's deputy spokesman, said earlier this week.
|The Kitty Hawk will stay off Taiwan |
until after the vote [AP]
The United States has urged Taiwan and China to avoid steps that would change the status quo in cross-strait relations and to resolve their differences through dialogue.
A recent report by the Pentagon said China was continuing to focus the bulk of its military build-up along its side of the strait separating Taiwan from the mainland.
"China's near-term focus on preparing for contingencies in the Taiwan Strait, including the possibility of US intervention, is an important driver of its [military] modernisation," the report said.
It said that while the region remained stable, China was adding more than 100 missiles a year to the estimated 1,000 it already has pointing at the island.
With hundreds of thousands of troops based in the area and hundreds of aircraft, the report said the balance of power was increasingly growing in China's favour.