Iranian troops crossed a key waterway in southern Iraq today and said that they had taken over a strategic island and inflicted damage on the city of Basra -- claims denied by Iraq.

Western military analysts, speaking by telephone from Baghdad, said the Iranian assault may be the first wave of a major offensive.

A statement issued in Tehran tonight said that Iranian forces seized a "wide area of sensitive and strategic regions" west of the disputed Shatt al-Arab estuary and near Basra, Iraq's second-largest city, and that the territory remained under Iranian control.

Iran said its forces captured Umm Rassas, an island that it said contained important Iraqi oil installations.

"Basra has sustained considerable damage, and a large number of enemy soldiers have been routed and a considerable number have been captured by the forces of Islam," Tehran radio said. "Areas that have been conquered and the booty obtained are sizable."

Iraq said that its forces were counterattacking along a broad front in the region north of Basra at the northern end of the Persian Gulf and had crushed "the main weight" of the offensive. But a statement issued in Baghdad acknowledged that the Iranians had gained "a shaky foothold in some forward areas" of Iraqi territory.

Radio Baghdad quoted a correspondent for the state-run Iraqi News Agency as saying the Iranians paid a "dear price" in the assault and Iraqi forces were pursuing the remaining Iranians. The correspondent said bodies of dead Iranian soldiers were floating in the waterway.

There was no independent confirmation of either side's report.

The late winter is favored by the Iranians for attacks because rainfall leaves the marshy region of southern Iraq a quagmire that makes it difficult for the Iraqis to use their superiority in tanks and other armor to full advantage. Analysts said Iran has had 250,000 troops massed on the southern war front for several weeks.

It is estimated that the 5 1/2-year-old war, in the oil-rich region of the Persian Gulf, has caused half a million casualties. During an offensive in the same region last year, Iran lost about 30,000 dead in one week; Iraqi deaths were placed at 10,000.

While the Iranian offensive was turned back last year, western diplomats said Iran appears to be far better prepared now.

According to the war communiques, the Iranians appeared to have attacked on two fronts through the Hawizah marshes beginning last night, the eve of the 7th anniversary of the Islamic Revolution that toppled the shah and brought Ayatollah Ruhollah Khomeini to power.

After last year's defeat, the Iranians built three roads into the marshes and began fortifying the islands there. They appear to have established artillery emplacements to provide cover for the offensive, as well as using the islands as jumping-off points for reinforcements and heavy equipment.

The Iranians severed the main north-south Iraqi highway during the fighting last year but were equipped with only personal weapons. Iraq, which holds a considerable superiority in tanks and warplanes, was able to crush the advance before the Iranians dug in.

Western experts said Iraq's strategy appears to be aimed at allowing the Iranians to make initial advances, onto high, dry ground where Iraq can use its armor.

To mark the anniversay of the revolution, Khomeini, 83, made a rare public appearance in Tehran. Speaking to foreign Moslems, he said that "today there is no government that would not like Iran to show some flexibility toward it, and that is true for the United States, the Soviet Union and France. They are the ones who are isolated."