Secretary of State Alexander M. Haig Jr. declared yesterday that the 20-month war between Iran and Iraq is threatening to spread to nearby states of the strategic Persian Gulf, and he announced that the United States plans "a more active role" in backing a negotiated settlement.

In a Chicago address, he declared that "now is America's moment in the Middle East" and promised greater U.S. diplomatic activism to advance Palestinian autonomy negotiations and to shore up the sagging cease-fire in Lebanon.

At the same time, the State Department announced the sale to Israel of 75 more advanced F16 fighter bombers for $2.7 billion.

The transaction would double the number of F16s in the Israeli inventory and, in the Pentagon view, maintain Israeli military superiority.

Haig's statements about the Persian Gulf war came amid intensified international struggle and maneuvering generated by the increasingly clear victory of Iran's military forces and deepening worry about the extension of Ayatollah Ruhollah Khomeini's Iranian revolution to conservative Arab states, including Saudi Arabia.

The Reagan administration has come under growing pressure from allies of both sides to take a less passive posture about a war that, according to Haig, "has brought the region into great danger" and "may lead to unforeseen and far-reaching changes in the regional balance of power, offering the Soviet Union an opportunity to enlarge its influence in the process."

Haig's declaration to the Chicago Council on Foreign Relations, stressed continued U.S. neutrality in the conflict. Charging that the war is fueled by Soviet arms supplied to both sides, Haig said Washington has refused and will continue to refuse to permit U.S. controlled military equipment to be supplied to either side.

According to official sources, Washington has rejected several feelers from Jordan to help Iraq by permitting indirect shipment of U.S. military gear. The sources denied reports that Egypt, which has also taken the Iraqi side and is said to be selling old Soviet-supplied weaponry, has asked to transship U.S. weaponry to Baghdad.

In a related but perhaps coincidental action, the administration has decided to go ahead with the controversial sale of six civilian transport aircraft to Iraq, overriding the objections of 35 senators and 64 House members.

The action was the first major commercial step toward that country since administration action three months ago to remove Iraq from the list of nations supporting international terrorism.

On the other side, Israeli Defense Minister Ariel Sharon, speaking on NBC's "Today" program, publicly confirmed for the first time yesterday that his country has been supplying military gear to Iran. Sharon said the Israeli supply is "very small," and other sources described it as tires and spare parts for Iran's American-made F4 Phantom jets.

U.S. officials said Israel has denied that any U.S.-supplied equipment is involved in its sales to Iran. Among the strange bedfellows supplying Iran, in addition to Israel, are North Korea, Syria and Libya, according to U.S. sources.

Haig, saying neutrality in the Iran-Iraq war is not the same as indifference, declared that U.S. friends and interests in the area are endangered by a continuation of fighting. The United States supports "constructive efforts" to bring an end to the fighting and supports withdrawal of forces behind international boundaries, he said.

"In the weeks ahead, we shall take a more active role with other concerned members of the international community as efforts are intensified to end this tragic war," Haig said. He supplied no details, but sources said a greater degree of U.S. diplomatic activity was envisioned in support of international mediation efforts already under way.

Haig received a personal appeal Tuesday from Jordan's Crown Prince Hassan for U.S. diplomatic leadership to contain Iran and create a peace zone in the Persian Gulf. Hassan told reporters yesterday of his fears that Iran will move to assert control over Islamic holy places within Iraq and seek to "resurrect Cyrus' empire" through a broad swath of the Middle East.

Haig's Mideast address, an unusually extensive statement of U.S. policy in that region, followed criticism that administration inactivity had brought about a loss of diplomatic momentum and growth of new dangers.

Regarding the Israeli-Egyptian negotiations on autonomy for Palestinians on the West Bank and Gaza, an uncompleted part of the Camp David accords, Haig said the United States will "intensify our effort to achieve success" in the period ahead. Sources said a major push is likely in late summer or fall, probably including meetings of Haig and his Israeli and Egyptian counterparts.

Appealing anew to Palestinians and Arab states to join the discussions, Haig said "refusal to participate in the talks by those most affected by the conflict risks the loss of the best chance for the achievement of a lasting peace."

In remarks that seemed aimed at recent Israeli policy in the occupied West Bank, Haig criticized "unilateral actions by any party that attempt to prejudge or bias the final outcome of the [automony] process."

Testifying on Capitol Hill, Assistant Secretary of State Nicholas A. Veliotes said the United States has not resolved the Israeli-Egyptian deadlock on the site for the next session of autonomy negotiations. But he was confident the hurdle will be overcome.

Regarding the situation in Lebanon, Haig announced that special U.S. emissary Philip Habib will return "soon" to the area where he helped arrange last summer's truce. Haig suggested that this time Habib will seek to go beyond the "fragile cease-fire" to present U.S. ideas, with cooperation of other states, to advance Lebanon's territorial integrity and a strong central government there.