The "package deal" being arranged here for the release of American hostages in Tehran has been delayed by the failure of Iranian authorities to set a time for freeing the captives, informed sources said today.

Sources said Iranian President Abol Hassan Bani-Sadr has agreed in principle to release the Americans, but has hesitated to fix a date until he can guarantee cooperation from the militants occupying the U.S. Embassy in Tehran.

Without such a firm commitment, U.S. officials refuse to accede to the Iranian demand for an international commission to be sent to Tehran to inquire into "crimes" of deposed Shah Mohammad Reza Pahlavi, sources said.

Despite snags over the timing of a release, U.N. Secretary General Kurt Waldheim continued to work on the composition of the panel.

The U.N. chief is scheduled to name the commission within 48 hours even if the question of the timing of the hostages release remains unresolved. It is believed that just unveiling the panel would be seen as a sign of good faith in Iran and strengthen Bani-Sadr in dealing with the militants.

Four men are known to have been asked to join the commission, but sources said that Waldheim has encountered problems landing at least one more member who would agree to serve and be acceptable to both sides.

Thus far, the panel is said to include Louis Pettitti of France, a judge on the European Court of Human Rights; Mohamed Bedjaoui, Algeria's ambassador to the United Nations; Venezuelan Andres Aguilar of the Inter-American Human Rights Commission, and chief judge and first president of Bangladesh, Abbu Sayeed Choudhury.

The commission's mandate is expected to be limited to gathering facts and publishing them without casting moral judgments. Two weeks have been set aside for the panel's inquiry in Tehran, according to sources.

Although Waldheim has received general assurances that the hostages would be freed after the inquiry begins, he has not been given a more specific timetable in his daily talks with Bani-Sadr and Iranian Foreign Minister Sadegh Ghotbzadeh.

Under one plan said to be under discussion, the hostages would remain in Tehran during the inquiry and be freed once the panel leaves Iran. Some of the Americans now hostage could be called as witnesses.

But other sources say the approximately 50 Americans -- who have been in captivity since the militants stormed the U.S. Embassy Nov. 4 -- would only be sent home after the commission completed its report to Waldheim and published its findings.

Setting a time for the release has always been the stickiest negotiating issue because of the political dynamics in Tehran, where Bani-Sadr competes for influence with the young embassy occupiers.

The picture was further clouded today when Bani-Sadr failed to meet with the militants, who say they will release the hostages only if the deposed shah is returned to Iran or if they receive orders from Ayatollah Ruhollah Khomeini.

Khomeini, 79, the ailing revolutionary leader, has remained silent on the Waldheim mediation effort, although Bani-Sadr said earlier this week that Khomeini gave his blessings to the president's plan for ending the crisis.

A U.N. official warned that negotiations are "full of traps and possible breakdowns," unless Bani-Sadr can "assure transfer" of the hostages from the hands of the militants to a third party, such as the International Red Cross.

Waldheim has moved in near total secrecy to avoid upturning Bani-Sadr's mission.

Overwhelmingly elected as Iran's first president last month, Bani-Sadr has gradually chipped away at the influence of those holding the hostages by calling them reckless children and accusing them of acting as "a government within a government."