Iran said its forces broke through Iraqi defenses east of the southern port city of Basra Monday, killing or wounding 4,000 Iraqis nearly three weeks after the Iraqis drove them out of the region.

Iran's official Islamic Republic News Agency, monitored in Nicosia, said fierce fighting was raging in the Salamcheh region, with the Iranians "holding the upper hand" in a two-pronged offensive.Iraq said the Iranians had crossed the border in what it called a "reckless assault" at the deserted police posts at Bubiyan and Kut Suwadi 15 miles east of Basra.

An Iraqi military spokesman, who cannot be identified under military regulations, told reporters in Baghdad that the Iraqis' 3rd Army Corps "absorbed the momentum" of the offensive and contained it.

He acknowledged fighting was still going on east of Fish Lake, an area flooded by the Iraqis several years ago to block Iranian thrusts.

The spokesman said the Iraqis were preparing a counteroffensive "that will teach the enemy a good lesson." He did not elaborate.

There was no independent confirmation of either sides' claims. The two nations rarely allow correspondents or other observers into the combat zones.

IRNA said Iranian troops punched through Iraqi defenses, destroying 30 Soviet-made tanks, decimating three infantry brigades and then overrunning three artillery battalions, setting ammunition dumps on fire.

IRNA correspondents with the Iranian assault force reported that thick black smoke from the burning Iraqi depots billowed over the battlefield.

The agency reported that several hours after the offensive was launched, a half-dozen other Iraqi infantry brigades had suffered 25 to 40 percent casualties. It made no mention of Iranian losses.

The agency said the battlefield was littered with knocked-out tanks and armored vehicles and that more than 700 Iraqis were captured, including several senior officers.

IRNA said Iranian artillery was pounding Iraqi positions behind the lines to block reinforcements reaching the hard-pressed frontline units.

The offensive was the first major Iranian operation announced in the strategically important southern sector since May 25.

At that time, Iraqi soldiers pushed the Iranians out of a hard-won bridgehead east of the southern port city of Basra in a lightning offensive that apparently caught the Iranians by surprise.

The Iranians had seized marshlands east of Basra with severe losses in early 1987 and battled to within six miles of the city before they were blocked by Iraqi defenses.

The loss of the Basra bridgehead was Iran's second battlefield defeat in five weeks.

The Iraqis had recaptured the Faw Peninsula south of Basra on April 18, ending two years of Iranian occupation.

The Iranians, jolted by loss of their two main footholds on Iraqi soil, had been expected to launch a retaliatory offensive in an attempt to regain military initiative in the nearly 8-year-old Persian Gulf war.

Iran's leader, Ayatollah Ruhollah Khomeini, appointed Parliament Speaker Hashemi Rafsanjani acting commander-in-chief June 2.