Iranian troops have decimated three Iraqi divisions around the Dezful border area in an offensive that has left Baghdad's invading military forces on the verge of collapse, U.S. officials said last night.

Intelligence reports reaching Washington said 20,000 Iraqi troops have been battered by Iranian attacks that came in three big waves, one last Monday, the second Wednesday and the biggest of all this past weekend.

The reports appeared to confirm Iran's claim yesterday that it had made major advances in the on-again, off-again war that broke out on Sept. 22, 1980.

Iraq's forces had quickly pushed 40 or more miles into Iran along a hundred-mile front at the start of the war, capturing or damaging several provincial cities and major Iranian oil facilities. The war climaxed years of border disputes between the two countries and, at the beginning, Western analysts predicted a quick Iraqi victory.

But Iran's Army has shown considerable improvement in recent months, and, in an indication of its new confidence, Tehran's revolutionary government Sunday allowed a group of American reporters into the country for the first time since January 1980, "to see the victories of the Islamic army."

U.S. officials said the toll of killed and wounded has been extremely heavy as Iranian artillery, fighter bombers, armor and waves of infantry concentrate their fire on a stretch of border country near Shush, 20 miles southwest of Dezful, in Khuzestan Province. Iran said Sunday that it had killed 8,000 Iraqis and wounded 12,000 in the past week.

While declaring they still do not have reliable figures on killed and wounded, these officials said there is hard evidence that about 20,000 Iraqi troops have been put out of action.

"It's the biggest battle of the war," said one official with access to the U.S. government's top secret intelligence. "The Iraqis look like they're on the verge of collapse."

The Iranians have succeeded in enveloping Iraqi forces in the Shush-Dezful sector, about 40 miles inside Iran, with pincer movements, officials said. Although both sides have suffered heavy casualties, the Iranians were described as much better off than the Iraqis as far as being able to send reinforcements to the battle area.

However, U.S. analysts said that the Iranians have not brought in the trucks, armor and supply it would take to drive deeply into Iraq. Therefore, Iraqi President Saddam Hussein could lose this biggest battle of the war without suffering total defeat.

Even so, analysts studying the intelligence doubted that Iraq would have enough strength left to mount any kind of counteroffensive soon.

Iraq has acknowledged only that there is a major battle under way near Shush. On Friday, just before Iran's big push, Iraq's press office here issued a statement claiming that "around 16,000 enemy men were killed during the past five days of fierce battles in the Shush-Dezful sector."

The statement said Iranian forces had tried to catch Iraqi units between pincers and said that "the Iranians thought that they could surround Iraqi forces" in the sector by sending in a "very large number of men."

U.S. officials studying the intelligence reports on the Iranian buildup for the offensive expressed astonishment last night that Iraqi forces did not organize a better defense, especially since Iran's Ayatollah Ruhollah Khomeini had announced a push was coming.

Iran committed more than 50,000 troops to the offensive, intelligence sources said.

"It has been a combined arms offensive," said one U.S. analyst in declaring that Iran seems to have marshaled impressive firepower to support what one high official termed "human-wave attacks" by a seemingly inspired Iranian infantry.

The three Iraqi divisions that have been smashed, U.S. officials said, total between 20,000 and 30,000 men. They are the 3rd and 10th armored and the 1st mechanized divisions.

[Radio Tehran said yesterday that all Iranian forces along the southern and western part of the border with Iran had been put on full alert "to receive the next order to launch the final sweeping attack on Iraqi troops," United Press International reported. The attack was planned for "H-hour after midnight tonight ]Monday[," the radio said. It added that the offensive will be directed by the new joint military command, just created in a merger of commanders of the regular army and the revolutionary guards. The army and the guards previously carried out separate military operations.]