The shipment did not include nuclear materials, but the error is particularly sensitive because China vehemently opposes US arms sales to Taiwan. Four of the cone-shaped fuses were shipped to Taiwanese officials in late 2006 instead of helicopter batteries that they had ordered.

Despite quarterly checks of the inventory, defence officials said they never knew the fuses were gone. Only after months of discussions with Taiwan about the missing batteries did the Pentagon finally realise, late last week, the gravity of what had happened.

On Taiwan, officials of the American Institute in Taiwan, which serves as the US government's official presence on the island, discussed the mistake with appropriate Taiwan authorities and expressed appreciation for their help in bringing the problem to US attention, the State Department said on Tuesday in a statement. It also said the officials explained US efforts to resolve the situation.

Once the error was discovered, the military quickly recovered the four fuses. How it happened, and whether the incident constitutes a violation of any treaty or agreement governing international sales of missile technology, were lingering questions.

Second nuclear-related mistake involving US Air Force in months
It was the second nuclear-related mistake involving the US Air Force in recent months. Last August an Air Force B-52 bomber flew across several US states carrying six cruise missiles armed with nuclear warheads.

After the flight from an Air Force base near Canada landed at one near the Gulf of Mexico, the deadly missiles were discovered. Even the pilot and crew had been unaware they had nuclear arms aboard.

Considering the two incidents, White House press secretary Dana Perino was asked on Tuesday whether President George W. Bush still has confidence in Air Force leaders. 'Yes. Yes, he does,' she said.

At a hastily called news conference, Mr Ryan Henry, the No. 2 policy official in Defence Secretary Robert Gates' office, called the mistake intolerable and said Bush as well as Chinese leaders were informed of the mistake.

'I cannot emphasise forcefully enough how strong the secretary feels about this matter and how disconcerting it is to him,' Mr Henry told reporters. He added that in an organisation the size of the Defence Department there will be mistakes, but 'they cannot be tolerated in the arena in strategic systems, whether they are nuclear or only associated equipment, as was in this case.'

US policies on arms sales to Taiwan not changed
In a comment directed at the Chinese concerns, Mr Henry said the error does not suggest that US policies on arms sales to Taiwan have changed.

Mr Christopher Hill, US assistant secretary of state for East Asia, said the Chinese have been briefed by US officials in Washington and Beijing.

'We told them what happened, because this was obviously an accident,' Mr Hill said after a speech in Washington. 'We've been very transparent with them about what happened.'

Taiwan, which split from China amid civil war in 1949, is the most sensitive issue in US-China relations. Chinese officials repeatedly complained about US arms sales to Taiwan during meetings with Mr Gates in Beijing last fall. The United States insists it provides only weapons that would allow Taiwan to defend itself.

Beijing claims Taiwan as its own and has threatened to attack should the self-governing island make its de facto independence formal. Washington has hinted that it would go to war to protect Taiwan.

The nearly two-year saga of the fuse shipment began in August 2006.

According to Air Force Secretary Michael Wynne, the fuses, contained in four large shipping containers, had been sent from an Air Force base in the Western United States to a Defence Logistics Agency warehouse in a nearby state. The containers apparently ended up in an unclassified area, rather than a classified section where they belonged.

In August 2006, the cylindrical containers, which measured almost 33 inches high and almost 19 inches in diameter, were sent to the government of Taiwan.

There they were placed in storage, US officials said.

Sometime in 2007 - exactly when is not clear - Mr Wynne said Taiwanese authorities notified US officials that they did not get the batteries they had ordered. Discussions ensued for months, during which, 'we, on our side, thought we were talking about different sorts of batteries. There was an effort to resolve and to reimburse them,' said Mr Henry.

Finally, late last week, U.S. military officials realised what had been shipped to Taiwan and worked immediately to get the fuses back. They have now been recovered.

Transparency
At the White House, Ms Perino said Mr Bush 'appreciates that they are taking action, and that there is a full investigation under way.'

Mr Henry said if the incident is a violation of any treaty or agreement, it was unintentional.

'We are being totally transparent. We have corrected the situation,' he said. 'The United States stands up to its treaty obligations and we're dealing with this in the most straightforward manner we can.'

Mr Gates has ordered a full investigation, and in a memo Tuesday he put Navy Adm. Kirkland H. Donald in charge and asked that Adm. Donald report back with an initial assessment by April 15.

Mr Henry said an examination of the site in Taiwan where the components had been stored after delivery indicated that they had not been tampered with. He said the components were 'quite dated,' as part of a system designed in the 1960s.

The Chinese Embassy did not immediately respond to a request for comment. A Taiwan official said on Tuesday that the island's diplomats in Washington typically do not comment on Defence Department matters.

The fuses were manufactured for use on a Minuteman strategic nuclear missile and are linked to the triggering mechanism in the nose cone, but they contain no nuclear materials. -- AP