Austrian Chancellor Bruno Kreisky plans to lead a European Socialist mission to Iran today, becoming the first Western head of government to visit Tehran since the American Embassy was seized last November.

Yesterday's announcement of Kreisky's visit came as a Syrian U.N. envoy, Adib Daoudi, arrived in Tehran to try to revive a suspended U.N. commission on Iran. The five-man commission was formed as part of a deal for the release of the American hostages, but left Iran in March when the arrangements broke down.

Despite the two visits, there were signs of a toughening Iranian position on the hostages following an International Court of Justice verdict yesterday ordering Iran to free them and pay damages to the United States.

Reacting to the verdict, the state-run Iranian radio said that "without doubt" all the captives would be tried as spies.

Meanwhile, the Swiss Embassy in Tehran is contacting all American citizens still in Iran to urge them to leave the country within the next two weeks, Swiss officials said. They said the embassy, which represents U.S. interests in Iran, was merely renewing, at Washington's request, a standing recommendation dating back to November.

At least 200 Americans are thought to be still living in Iran, many of them married to Iranians. The Swiss Embassy is offering quick visas to the Iranian families of Americans and in some cases paying plane fares for those who wish to leave, Swiss officials in Tehran said.

Kreisky is to travel to Iran on behalf of the Socialist International with former Swedish prime minister Olof Palme and Spanish Socialist Party leader Felipe Gonzalez. Spokesmen said the group hoped to meet President Abol Hassan Bani-Sadr.

Bani-Sadr, meanwhile, indicated plans to nominate Hassan Habibi, spokesman of the ruling Revolutionary Council, as Iran's new premier.

The president told a radio interviewer that he had now made up his mind who his choice would be for premier, and would present his candidate in three weeks to the newly elected parliament, which has to vote on the appointment.

Asked about a press report that Habibi -- who also is minister of higher education and culture -- had already become premier, Bani-Sadr said: "Will be chosen. You should change 'has become' to 'God willing, will become.'" He did not elaborate.

The official Pars news agency earlier quoted Habibi as denying the report that he had already been named prime minister.