In a maneuver that could strike the most serious blow yet to Iran in its six-day-old war with Iraq, Iraqi forces were said to be approaching the Iranian City of Dezful, a vital rail junction 45 miles inside Iran and at the northern tip of Iran's oil-rich Khuzestan Province.

Senior administration officials, reviewing the latest intelligence reports reaching Washington, indicated that the thrust toward Dezful could be the capstone of Iraq's military strategy.

Iraqi forces were estimated to be still another two or three days from being in a position to take the city. But if that happens, officials said, they would essentially isolate Khuzestan from the rest of Iran, cutting off rail and road links that are virtually the only way to the south from the mountainous regions surrounding the province.

Aside from containing most of Iran's oil fields, the province has always been a place where Iran feared internal rebellion since it is home to more than a million Arabs, who are of different ethnic origin than Iranians.

Iraqi forces, according to information made available here, have the key Iranian oil refinery town of Abadan to the south surrounded and were fighting inside Khorramshahr, Iran's biggest port.

Officials here said Iraq was in a militarily commanding position in those two places and thus was in a position, with the drive on Dezful, to put a stranglehold on the whole province.

It is widely believed that with these strategic points in hand, Iraq would accept a cease-fire and, at the same time, see itself as having succeeded in bringing both extreme pressure on and humiliation of Iranian leader Ayatollah Ruhollah Khomeini.

Administration officials said that Iraq has about 80,000 troops in the front-line border regions as opposed to only about 30,000 Iranian soldiers near the front.

The Iranians are in four divisions -- two infantry and two tank units. But officials here said only two of the Iranian divisions had actually been involved in combat. Iran has a much larger Army, but officials said thus far Iran had not moved major new forces up to the fighting front.

Some officials speculated that Iran may have been reluctant to move divisions from their garrisons because of concern about internal unrest. On the other hand, Iran could be planning a counterattack if there are sufficient supplies, or may be having trouble coordinating its armed forces.

In Washington, the expectation seemed to be increasing that the war would probably be winding down in the next several days as both sides, but especially Iran, felt the pinch of dwindling supplies.

Officials here said both sides are slowly losing and using up their air forces in attacks that are characterized more as "mindless zooming" rather than well carried out air raids.