Encouraged by a de facto Iraqi cease-fire in the Persian Gulf, Secretary of State George P. Shultz will ask Iraqi Foreign Minister Tariq Aziz here Monday to continue holding fire against gulf shipping while the United Nations seeks Iran's cooperation to deescalate or end the war, State Department officials said yesterday.

Shultz' meeting with Iraq's senior diplomat, who is also deputy prime minister in the Baghdad government, opens a new phase of U.S. diplomacy in search of solutions to the Iran-Iraq conflict. Aziz arrived in the United States late last week at the invitation of U.N. Secretary General Javier Perez de Cuellar, who is seeking Iraqi and Iranian compliance with last Monday's Security Council resolution demanding an end to the nearly 7-year-old war.

Iran has verbally lambasted the U.N. resolution, which calls for a cease-fire and return to prewar boundaries and lays the basis for international sanctions against parties which do not comply.

Iranian Foreign Minister Ali Akbar Velayati, who was also invited by Perez de Cuellar, did not come to New York.

Nevertheless, a senior State Department official noted that Iran has not formally rejected the security council resolution and said he doubts that it will.

The official said that "in logic, there is a fair chance for a de facto cease-fire" in which the war pauses or sharply diminishes in intensity at least for some months. The United States, he said, is seeking to advance this aim.

U.S. diplomatic activity centering on the U.N. resolution has intensified as U.S. naval forces began protecting reflagged Kuwaiti oil tankers, a change in Washington's posture that has been criticized in Congress. U.S. policy toward Iran in a different context has been the subject of the highly publicized Iran-contra hearings since May 5.

Iraq's attacks on gulf shipping, which began three years ago, were designed to put economic and military pressure on Iran, which is three times its size. Both countries have depended on oil exports to finance the war, and most of Iran's must pass through the gulf.

A series of statements by Iran, including a speech by President Ali Khamenei last Monday, said Tehran will not attack shipping of other nations as long as its vessels are not hit. But Khamenei and others threatened to press the attack against gulf shipping, including reflagged Kuwaiti vessels bearing the U.S. flag, if Iranian ships come under fire.

According to State Department data, Iraq mounted air attacks on Iran-connected shipping in the gulf every four to five days between June 20 and July 15 but has not attacked shipping since then, perhaps in deference to the U.N. resolution. Officials said some ground action on both sides has continued, especially in Kurdistan.

Iran's latest attacks on shipping were a July 13 gunboat attack on a French freighter and a July 9 gunboat attack on Liberian-owned oil tanker. A reflagged Kuwaiti tanker, the Bridgeton, under U.S. flag and naval escort, struck a mine early Friday. A U.S. official said that, while there is no proof or certainty, the likelihood is that the mine was placed by Iran in a gulf ship channel a day or two before the Bridgeton struck it.

Iraq has "welcomed" the U.N. resolution but said it would not agree to terminate attacks on Iran-bound shipping unless Iran agrees to the full cease-fire and return to international boundaries that are part of the U.N. resolution.

One U.S. objective, according to administration officials, is to persuade Iraq to continue its cease-fire for at least long enough to give Perez de Cuellar time to make a full effort to persuade Iran to follow suit. The State Department has not given up on the possibility of Iranian cooperation, either acknowedged cooperation or an unacknowledged de facto cease-fire or reduction in hostilities. Others in the administration, however, said that any Iranian cooperation is unlikely and that the key effort must be to intensify pressure on Iran for its lack of compliance with the U.N. resolution.

Within a few weeks, an administration official predicted, it will be clear that Iran is ignoring the U.N. resolution and therefore it will be time to ask the U.N. Security Council to vote enforcement sanctions, such as an embargo on sales of arms. Even if this is accomplished, the pressures will take their toll only gradually, the official said.