Signs that Iran showing some flexibility on UN cease-fire plan. UN PEACE BID

Diplomats at the United Nations say that talks with Iran on ending the Gulf war have taken an important step forward. Iranian President Ali Khamenei, these sources say, told the UN Secretary-General Tuesday evening that Iran was prepared to agree to a cease-fire simultaneous with the establishment of a committee to investigate the causes of the war. This represents a significant concession as it is the first time Iran has agreed to a formal cease-fire, though with conditions.

The Secretary-General is to discuss Iran's position with the foreign ministers of the five permanent Security Council members - the United States, Soviet Union, Britain, France, and China - at a luncheon meeting today.

Soviet Foreign Minister Eduard Shevardnadze, in his speech to the General Assembly on Wednesday, said the Council should take steps to assess the blame for the war ``within a clearly established time frame,'' casting doubt that the Soviet Union would support any move toward sanctions at this stage. Mr. Shevardnadze and US Secretary of State George Shultz met at the UN yesterday and President Reagan issued a statement calling on the Soviets to cooperate on a sanctions resolution. The Council could hold sanctions discussions, diplomats say, to keep the pressure on Iran.

Iranian officials had indicated to Secretary-General Javier P'erez de Cu'ellar during his recent visit to Tehran that they were interested only in an undeclared cessation of hostilities while the investigating committee carried on its work. They said they would only accept an official cease-fire once the committee had reached its verdict.

Though President Khamenei indicated no shift in Iran's position during his speech to the UN General Assembly Tuesday morning, he apparently offered the change in the private meeting he had with the Secretary-General that evening, diplomats say.

On Wednesday morning, UN officials began working sessions with Iranian Deputy Foreign Minister Muhammad Javad Larijani and Foreign Ministry Director-General Jaafar Mahallati to negotiate issues relating to the commmittee of inquiry and cease-fire.

Progress was made, diplomatic sources say, in defining Iranian views on the composition of the impartial investigative body on the causes of the war. Iran has agreed that the committee can be composed of three to five ``independent figures'' - individuals not serving any government - who are highly qualified in international law and history.

Iran resists the inclusion of diplomats. It fears diplomats would be subject to political pressures to render a split decision in allocating responsibilities for the conflict - holding Iraq responsible for launching its invasion of Iran on September 22, 1980, but also charging Iran with prolongation of the war by refusing negotiate terms of a settlement with Iraq and pushing for the downfall of the Iraqi government.

Iraq, in recent weeks, has publicly suggested that the investigative body be juridical, and has indicated a strong preference for the World Court.

According to the plan currently under discussion, each side will nominate one or two members. Once the commission is formed, it is expected to conclude its work relatively quickly - within one to three months, with a margin of allowance for delays from both sides.

This proposal will have to be discussed with Iraq and be put to the full 15-member Security Council.