The Reagan administration considers Taiwan a substantial asset to its emerging Asian policy, but there is not urgency to decide whether to sell that island bastion an advanced U.S. fighter plane, a senior State Department official said tonight.

The official, briefing American reporters under background rules often used during travels of secretaries of state, said Taiwan is "a rather impregnable aircraft carrier in a vital sea lane" and thus is an asset of considerable importance in the U.S. view.

But the senior official said he sees "no urgency . . . at all" to make the controversial and complex decision soon about selling an advanced fighter plane to Taiwan. Taiwan authorities are pressing for a U.S. decision to supply the aircraft, but Secretary of State Alexander M. Haig Jr. was told last week in Peking that such a sale would have extremely serious consequences for U.S. relations with China.

A decision will be made on the bsis of the need for the weapons and the purposes they would serve, reporters were told tonight.

Haig spent three days last week in Peking, where he made known the Reagan administration's decision in principle to open an arms sale relationship with China. Haig then flew to a foreign ministers' meeting of the Association of Southeast Asian Nations in Manila. He flew here from Manila today for a foreign ministers' meeting of ANZUS, the alliance of Australia, New Zealand and the United States.

Washington has not yet reported to Taiwan on Haig's meetings in Peking but intends to do so soon, reporters were told.

The policies toward China, Southeast Asian countries, Japan and other Asian states set forth by Haig during this trip were depicted as a new phase of U.S. policy in a region that has seen American triumph in World War II, a hard-fought armistice in Korea and defeat in Indochina.

All along Haig's route, according to the senior official, it was made clear to friendly nations that the United States intends to play a strengthened role from the basis of increased military assets in the area. The planned increase in U.S. nval power, for example, will permit the United States tp patrol the Indian Ocean without drawing ships away from the western Pacific area, as it has been forced to do in recent months.

If the United States is prudent, it was said in the briefing here, it may be able to foster what is seen as a naturally emerging consensus in the region to work against Soviet efforts to bring about change through use of military forces.

The senior State Department official strongly disagreed with suggestions that the closer U.S. relationship with Peking, Moscow's archrival in the communist world, could encourage the Soviet Union to invade Poland.

The greatest incentive to the Soviets not to invade, the official said, is the knowledge that "they're going to pay a price" if they do. The official said it is the consistency of U.S. policy worldwide that will make it clear to Soviet leaders "that they're going to have to pull in their horns whether in Poland, Afghanistan, Kampuchea [Cambodia] or Central America."

Moscow is "not particularly happy" and is "unsettled" by the Reagan administration's policies, according to this official.

The message that should register in Moscow, the official said, was that "the United States has finally awakened" after a period of "failure to stand up" to the Soviets and it is "time to reassess" in the hope of improving relations.

Regarding the potential U.S. arms sales to China, the official said the "key allies" of the United States knew in advance what was being done by the Reagan administration. The official denied that Southeast Asian nations, some of which consider China a long-range threat to their security, protested the shift in policy during Haig's meeting with their foreign ministers in Manila.