President Carter, vowing that Iran "will continue to pay an increasngly higher price" for holding American hostages, announced yesterday that Secretary of State Cyrus R. Vance will go to the United Nations today to argue the U.S. case for economic sanctions against Iran.

"I expect to see adequate support in the United Nations Security Council for our position," the president said in response to questions about whether the United States can muster the necessary nine votes for adoption of a sanctions resolution in the 15-nation council.

Despite Carter's words, administration officials later said privately they still do not know how the council will vote or whether the Soviet Union will block a sanctions resolution with a veto.

In essence, the officials said, the outcome will depend on what kind of resolution the United States tries to push through the council. They expressed confidence that a relatively general and qualified resolution would win the necessary backing; they also conceded that a toughly worded resolution could run into severe difficulties.

Vance's mission, the officials said, will be to take charge of the intense, behind-the-scenes U.S. lobbying campaign and gauge whether a consensus can be worked out on trade and other economic boycott measures that the United States regards as adequate to keep the pressure on Iran's revolutionary government.

Noting that U.S. Ambassador Donald McHenry already has had extensive consultations with the other council members, the officials expressed hope that an agreement could be worked out in time to hold a formal council meeting later today. But, they added, the precise timing of the formal council debate and vote is still not certain.

In an effort to make a sanctions resolution more palatable to some hesitant council members, the United States reportedly is willing to agree to a two-stage approach. Under this plan, the council would be asked to vote for sanctions on the explicit understanding that they would not be put into effect until Iran is given the chance to release the hostages by a specified deadline.

Carter, announcing Vance's dispatch to New York in a nationally broadcast statement after a day-long meeting with his top security advisers, reiterated his determination to use every available international forum in seeking a peaceful resolution of the seven-week confrontation with Iran.

"The United States reserves the right to protect our citizens and our vital interest in whatever way we consider appropriate in keeping with principles of international law and the charter of the United Nations," he said.

"But our clear preference is now, and has been from the beginning of this crisis, for a quick and peaceful solution through concerted international action."

The president concluded: "A thoughtful and determined policy which makes clear that Iran will continue to pay an increasingly higher price for the illegal detention of our people is the best policy to achieve those goals, and it is the policy I will continue to pursue."

Diplomatic sources said yesterday that the United Nations has been circulating a draft resolution that calls for an embargo by U.N. member nations of all Iranian goods carried in Iranian ships, except oil, and all Iran-bound exports except food and pharmaceuticals.

In addition, the draft resolution would forbid member countries to allow Iranian ships and planes into their territory and would block them from granting credit to Iran or shipping military spare parts there.

However, the draft resolution is regarded as a "talking paper" that sets out the maximum U.S. position for bargaining purposes. Any resolution that does emerge from the Security Council, the sources stressed, almost certainly would be much less strict.

The question of how much watering down would be acceptable to the United States has led to some division within the administration. Some officials, noting that the nation's principal allies among the major industrial nations have agreed to cooperate in putting a credit squeeze on Iran, have argued that a relatively bland resolution would be sufficient.

Others, however, see the resolution as important mainly as a way of underscoring to the Iranian leaders that they are isolated in world opinion. This argument would make the resolution as tough as possible to give it maximum symbolic impact.

Complicating the situation is the change on Jan. 1 in five of the council's 10 non-permanent seats under the U.N's routine rotation system. As a result, a vote delay beyond Monday would force the United States to mount a new lobbying effort with the altered lineup -- one that many U.S. officials think would be more resistant to sanctions than the present council membership.

Another big question mark is whether the Soviet Union, one of the five permanent members with veto powers, will veto a resolution as a gesture to gain influence with Iran and other countries of the Islamic world.

Although Washington has warned the Soviets that a veto would have serious consequences for U.S. relations, Moscow has not yet revealed what it will do. Now, the mystery surrounding Soviet intentions has been deepened by Soviet military involvement in the coup in Afghanistan Thursday and the new strains this has imposed on the ties between Washington and Moscow.

In addition to its own vote, the United Nations is considered reasonably certain of support from six other members of the present council: Britain, France, Norway, Portugal, Bolivia and Jamacia.

To get the necessary additional votes, the United States has concentrated its lobbying efforts on the three African members, Gabon, Nigeria and Zambia, and on China, which is also a permanent member. China is understood to have promised not to veto a resolution, but it has not yet made clear whether it would vote for sanctions or simply abstain.