Iraq served notice today that it was withdrawing from its 1975 peace agreement with Iran that ended three years of border hostilities. The move reflected seriously deteriorating relations between the two Moslem neighbors, as well as Baghdad's unease about sectarian religious dissent both at home and in other Arab nations.

Analysts here suggested Iraq was seeking to press its relative advantage against troubled Iran as revenge for the 1975 accord, when Shah Mohammad Reza Pahlavi extracted a high price for ending the Kurdish conflict, which was undermining Bagdad's authority.

In addition to further undermining regional stability, increasing tensions between Iran and Iraq could seriously threaten a major portion of the world's oil supply. Iran's oil-producing region is inhabited largely by Arabs who are subject to appeals from Iraq, while Iraq has a large population of Shiite Moslems whose loyalties constantly are being tested by Iran's militant Shiite clergy.

A major recurring theme among international oil analysts, is a serious disruption of supply caused by worsening relations between Iran and Iraq.

The Iraqi move -- in a statement released to a newspaper here, a traditional conduit for major Arab world pronouncements also underscored tensions with other neighbors. It revealed, in additions an apparent desire to thwart Syrian and Palestinian mediation efforts aimed at smoothing over misunderstandings between Iran and other Arab states and indicated the Baghdad government's return to its classic odd-man out role in Arab politics.

Iraqi Ambassador Abdel Hussein Muslim Hassan told the independent, daily An Nahar that Iran should "immediately" grant self-rule to its volatile Arab, Baluchi and Kurdish minorities, withdraw from three key Persian Gulf islands and return to entire Shatt-al-Arab Esturary to Iraq. p

Under the March, 1975 Algiers accord, Iraq granted Iran half of the long disputed estuary, which leads 120 miles from the Persian Gulf to the confluence of the Tigris and Euphrates rivers.

The two signatories also agreed to define other common borders and stop supporting subversion within the other state. That undertaking ended American, Iranian and Israeli aid to the late Mullah Mustafa Barzani's rebellion in Iraqi Kurdistan, which soon thereafter collapsed. The demands in the statement published in An Nahar, and the suggestion that Iran "voluntarily" agree to the amendments therefore, effectively gut the agreement.

The short-lived reconciliation between the warring Baath Party governments in Iraq and Syria collapsed this summer after less than a year, Damascus propaganda resumed its old accusations that Baghdad had sold out Arab interests in the 1975 accord.

Relations between Iraq and the Shiite Moslem clergy that now rules Iran began deteriorating in the fall of 1978, when Baghdad forced Ayatollah Ruhollah Khomeini to leave his Iraqi exile in the holy city of Najjaf. The ayatollah replaced the shah in February this year.

In recent months, relations have declined further as Baghdad and Tehran have accused each other of arming and otherwise encouraging their respective Kurds to revolt.

And khomeini's government has backed the shah's controversial 1971 occupation of the Arab islands of Abu Musa and Greater and Lesser Tumb near the strategic Straits of Hormuz. An estimated 60 percent of the noncommunist world's oil passes through the area.

Before the 1975 agreement, Baghdad claimed Iran's oil-producing Khuzestan Province, which originally was populated exclusively by Arabs and which the Iraqis called Arabestan. Support for the Baluchis, split between southeastern Iran and southwestern Pakistan, was also a standard pre-1975 Baghdad policy.

Also behind the Iraqi move was increasing uneasiness of the Sunni-Moslem government in Baghdad in dealing with its Shiite Moslem majority ever since Khomeini came to power.

There has been Shiite unrest in Kuwait and Bahrain, which also have Sunni minority governments, prompting fears -- not only in Baghdad -- that Khomeini aspired to export his Shiite-led revolution.

Shiite riots in June were said to have helped prompt Iraqi strongman Saddam Hussein to oust President Ahmed Hassan Bakr and purge leading party and government officials the following month. Low level urban guerrilla activity by Shiite dissidents has been reported in and around Baghdad and other Iraqi cities.

New President Hussein's espousal of such nationalist Iraqi themes such as the return of all the Shatt-al-Arab, backing for the Khuzistan Arabs and an end of Iran's occupation of the three islands appear aimed at widening his power base.

His seeming adoption of the "best-defense-is-an-offense" policy toward Iran has led him, according to specialists, to encourage the presence of some of the shah's former generals -- and former ambassador to Washington Ardeshir Zahedi -- in the border area.

Many of the shah's pretorian guard -- the so-called Javidan, or "immortals" -- were Kurds and the high officers' reported presence in the border area was thought to be connected with anti-Khomeini activity.

Analysts were struck by the rapidity of the Iraqi government's return to its traditional go-it-alone attitude.

Only a year ago Iraq reversed a decade of isolation by offering to end its feud with Syria and cooperate with other Arab governments in an effort to offset a separate peace between Egypt and Israel.

But Saddam Hussein's master plan soon came undone. Contributing factors were the Iranian revolution, the Iranian Kurds' uprising, and the ensuing nervousness about Shiite unrest.