RIYADH, Saudi Arabia - The United States said yesterday that it would withdraw all of its combat forces from Saudi Arabia by the end of this summer, marking a major shift in the American military presence in the Persian Gulf region.

Defense Secretary Donald H. Rumsfeld and Saudi Defense Minister Prince Sultan bin Abdul Aziz said at a news conference here that the fall of Saddam Hussein's government in Iraq means that the U.S. aircraft here no longer need to patrol the "no-fly" zone over southern Iraq and that their military mission is over.

But the presence of U.S. forces has long been an irritant for Saudi rulers facing strong anti-American sentiment, despite or because of a history of military cooperation that dates to World War II. While Saudi officials said they did not request the withdrawal, the action could lessen criticism of the government.

"This does not mean we requested them to leave Saudi Arabia, but as long as their operation is over, they will leave," Prince Sultan said. Both he and Rumsfeld said the military relationship is not ending.

"The cooperation between the two countries was going on before and will continue even after the war in Iraq," Prince Sultan said.

"It is now a safer region because of the change of regime in Iraq," Rumsfeld said. "The aircraft and those involved will now be able to leave."

Rumsfeld said the United States will "maintain a continuing and healthy relationship with the Saudis."

The news conference was broadcast on Saudi national television, announcing a pullout of troops that the Saudis have always been reluctant to acknowledge were here. In the Iraq war, Saudi Arabia refused to allow American journalists to join military units based here.

This year, Saudi officials said the departure of American soldiers would set the stage for a series of democratic reforms, including an announcement that Saudi men - but not women, at least initially - would begin electing representatives to provincial assemblies and then to a national assembly.

The terrorist attacks of Sept. 11, in which 15 of the 19 hijackers involved were Saudis, prompted members of both parties in Congress to urge wide-ranging reforms in the conservative kingdom.

About 100 U.S. planes remain at Prince Sultan Air Base, down from about 200 during the height of the Iraq war, along with as many as 10,000 American personnel. All will be gone by the end of August.

Rumsfeld said the American withdrawal is part of an effort to "refashion and rebalance" the U.S. military posture in the region after the victory over Hussein's regime in Iraq. The realignment could include bases in Europe, he said.

The major American presence dates to 1990, after Iraq invaded Kuwait. Most of the 500,000 coalition troops that ousted Iraqi forces from Kuwait massed in Saudi Arabia.

The United States then built an air operations center at the air base, about 50 miles south of Riyadh, finishing it in early 2001. American commanders moved their oversight of air operations in the region on Monday from here to a similar command center built at the al-Udeid base in Qatar shortly before the Iraq war.

The United States used the al-Udeid center during the Iraq war to coordinate military flights in Afghanistan and the Horn of Africa, said Rear Adm. Dave Nichols, deputy air commander for Central Command.

U.S. commanders have chafed for years about restrictions the Saudis put on the use of the base the Americans practically built from scratch.

Prince Sultan, for example, said before the wars in Afghanistan and Iraq that he would never allow his namesake base to be used for U.S. attacks on Arabs or Muslims. The Saudis tried to suppress news of the base's use in the Iraq war and limited the kinds of missions that could be flown from here to Afghanistan.

The Saudis also worked to stifle news about U.S. Special Forces' use of other airfields, such as the one at Arar near the Iraqi border. Even the amount of money that each nation has spent on the military partnership has been kept quiet, though U.S. commanders say the Saudis provided free fuel for American planes.

Life for the roughly 5,000 troops based here permanently has been difficult. The 1,600 Army troops operating Patriot anti-missile batteries that have protected the kingdom cannot officially discuss their mission because of Saudi sensitivities. Security restrictions bar troops from going into Riyadh except on official business. Few American troops get a chance to meet Saudi civilians.

Only about 400 American troops will remain in the kingdom, most of them based near Riyadh to train Saudi forces, U.S. officials said.

Nichols and other officials here said the Pentagon has not decided whether to keep the Prince Sultan base "warm" - that is, keep a skeleton crew here so the base could be quickly restarted in an emergency.

"Nothing's going to be torn down," Nichols said. "It'll remain wired, but most of the computers and whatnot will be taken out."