WASHINGTON (AP) - A U.S. missile sale to Taiwan includes a highly unusual condition: that they will not be turned over to the island democracy unless China threatens an attack, a senior U.S. defense official said Friday.

The arrangement is designed to meet a U.S. arms export pledge not to introduce new offensive military capabilities into Asia, where tensions between Taiwan and China are a source of growing U.S. concern.

Taiwanese pilots will train with the missiles, designated the AIM-120C Advanced Medium Range Air-to-Air Missile, or AMRAAM, at Air Force training ranges in the United States, the defense official said, speaking on condition he not be identified. Their F-16s will be updated with the weapon control software.

``It's a way for them to train up and be ready should the threat occur,'' without provoking China into accelerating its pursuit of a similar capability, the official said. China's air-to-air missiles are far less advanced, although a Pentagon report to Congress this summer said that later this decade China is likely to field a new missile that would give its growing fleet of new-generation fighters an AMRAAM-like capability.

The AMRAAM, in use by U.S. aircraft since 1991 and built by Raytheon Co., allows a fighter pilot to launch the weapon from beyond visual range of his target. It also provides a greater capability to attack low-altitude targets.

The Taiwan arms deal comes at a particularly delicate moment in U.S.-China relations, which were set back last year when the U.S. Air Force bombed China's embassy in Yugoslavia. Just last week, relations took an important step forward when the Senate approved legislation to normalize trade relations with Beijing.

China on Friday condemned the Taiwan deal and warned that Washington would bear unspecified consequences for defying Beijing.

China, which regards Taiwan as part of the motherland, strongly opposes U.S. arms sales to the island. Under the Taiwan Relations Act, the United States is committed to providing Taiwan with defensive arms.

The $150 million deal for advanced air-to-air missiles seems likely to complicate U.S.-China talks on missile proliferation which resumed this summer after China broke all military contacts with Washington following the embassy bombing. The United States wants to constrain China's exports of missile-related technologies, while Beijing insists that U.S. missile sales to Taiwan show Washington uses a double standard.

Anthony Cordesman, a national security expert at the private Center for Strategic and International Studies, said Friday the AMRAAM arrangement with Taiwan strikes a sensible balance between Taiwan and China.

``Holding AMRAAM in reserve says that with little or no warning the United States can give Taiwan air supremacy over the Taiwan Straits,'' Cordesman said. Taiwan is barely 100 miles from the mainland.

The unusual arrangement was not mentioned in a Pentagon announcement late Thursday saying it planned to sell 200 AMRAAM missiles to Taiwan as part of a $1.3 billion arms package. The Pentagon said the sale would augment and improve Taiwan's air defense capabilities without upsetting the regional military balance.

The U.S. official who discussed the AMRAAM sale said it was approved in response to China's efforts to improve its offensive military capability aimed at Taiwan. He said U.S. officials believe China is pursuing the purchase or development of its own air-to-air missile with a beyond-visual-range capability, but the United States insisted that Taiwan not take direct possession of AIM-120C missiles for the time being.

``We don't want to spur an arms race in the area,'' the official said.

Earlier this week the Pentagon announced that it was selling 100 AMRAAM missiles to Singapore. Those, too, will stay in the United States and delivered to Singapore only in the event of a military threat against it, officials said.

In addition to the AIM-120C air-to-air missiles, the Pentagon also agreed to sell Taiwan 71 Harpoon anti-ship missiles for $240 million, a package of 155mm howitzers and other munitions for $405 million, and an advanced communications system for Taiwan's field military forces for $513 million.