U.N. Secretary General Kurt Waldheim today launched a new effort to resolve the U.S. hostage crisis by diplomatic means, dispatching a high-ranking Syrian official, Adib Daoudi, to Tehran as his representative.

The move was approved by both Iranian Foreign Minister Sadegh Ghotbzadeh and U.S. Secretary of State Edmund Muskie, U.N. officials said.

"But there are no guarantees," one U.N. source added. "These talks are important, but only exploratory. You never know how they will develop."

The United Nations refused to specify the timing of the Daoudi Mission, but he is expected to confer with Waldheim in London or Geneva within the next 10 days, and then proceed to Tehran.

A U.N. spokesman said Waldheim had two lengthy telephone conversations with Muskie on the Daoudi mission and discussed it thoroughly with Ghotbzadeh in Belgrade last week.

in Washington, State Department officials said the United States welcomed the dispatch of Daoudi to Tehran, although they stressed that it was a Waldheim initiative rather than an American one. "The name of the game at this point is exploration," said a senior American official in explaining Washington's attitude.

[In Maine, Muskie questioned the timing of Waldheim's initiative, saying it could put "American expectations on a roller coaster."] But he added, "I'm willing to pursue anything that holds any possibility, promising or unpromising."

[Iranian President Abol Hassan Bani-Sadr, meanwhile, pointed to the Ghotbzadeh-Waldheim talks as a sign or progress toward resolving the crisis and appealed for an end to Western economic sanctions against Iran.]

Daoudi, a political advvise to Syrian President Hafez Assad, had served on a five-man U.N. commission of inquiry set up by Waldheim on Feb. 20 to investigate Iranian grievances and obtain the release of the American hostages.

But after 13 days in Tehran, where the panel became enmeshed in an internal struggle bttween Islamic militants and the government of Bani Sadr, the commission aborted it mission without seeing the hostages or issuing a

The object of the Daoudi mission, today's U.N announcement said, is to "discuss the resumption of the work of the U.N. commission of inquiry and the completion of its mandate, including its report, in order to solve the crisis between Iran and the United States."

It appeared unlikely, however, that a new U.N. initiative would find a more hospitable climate in Tehran since, if anything, the authority of the Bani-Sadr government has deteriorated in the last two months.

Asked what reason Waldheim had to believe that the Bani-Sadr government now has the power to carry out its end of a "package deal" arranged earlier for the release of the hostages, U.N. officials acknowledged that there were no sure answers.

"But parliament will meet in Tehran soon," one U.N. source said, "and a new prime minister will be selected. The country is moving in the direction of consolidation."

Daoudi's purpose is not to visit the hostages, a U.N. official said, but "if he has the opportunity, he would do so. He will certainly inquire after their well-being."

Washington Post correspondent Leonard Downie Jr. reported the following from London:

In last-minute pleas to Washington's European allies to delay or moderate economic sanctions against Iran, President Bani-Sadr has said that with their help a solution to the dilemma of the American hostages could be near.

In diplomatic contacts with European governments and an interview for publication in four leading European newspapers, Bani-Sadr said the crisis could not be eased by the imposition of tough economic sanctions by European Community foreign ministers in Naples this weekend. He said the problem could be solved through diplomatic negotiations or by the new Iranian parliament.

Bani-Sadr said, "Iran is determined to solve the problem of the hostages in an honorable, final and peacefful manner.

"I turn to Britain, to France, to Germany, as well as Italy," he said. "Suggest to us wht to do. Put forward concrete proposals. Act as mediator Do something. Instead of just condemning us, take notice that the hostage problem is making objective difficulties not only for us but also for you."

Asked what conditions are necessary for the release of the hostages now reportedly held at numerous locations scattered throughout Iran, Bani-Sadr said: "One above all: the guarantee that the United States will not intervene, either now or later."

Bani-Sadr said all Iranian leaders, including his fundamentalist clerical opponents, were beginning to understand that the Iranians themselves were becoming "the real hostages" and thus must find a way out of the crisis quickly. While Iran could survive sanctions economically by trading with other countries or going without, he said, "politically it would be a catastrophe."

He portrayed himself as Iran's chief defender against the "obscurantist political dictatorship" that he said Ayatollah Mohammed Beheshti, leader of the dominant, fundamentalist Islamic Republican Party in the new parliament, and others might try to establish in Iran.

"I am determined to destroy, I say destroy," Ban-Sadr told his interviewer, "the authoritarian power centers which are seeking to install a new dictatorship -- obscurantist, reactionary and facist -- by disguising it as the ideal regime for every Moslem."