"Iranian forces hit a border area called Haj Umran and then entered five kilometers (three miles) into Iraqi territory and hit the area of Lollan with heavy artillery with 180 shells targeting PKK positions," an Iraqi defense ministry statement said.

The shelling was the second military attack on the Kurdish guerrillas by Iranian forces in 10 days. The previous attack on April 20 left two guerrillas dead and another 10 wounded.

The Kurdish rebel group, Kurdistan Workers' Party (PKK), who have dug-in in Iraq's northern Kurdish-controlled area on the border with Iran and Turkey, have warned Iran not to interfere in their fight against Ankara's rule in southeast Turkey.

The leader of a group of PKK rebels, Rustom Judi, told AFP in an interview that Iranian forces have "no reason" to fight the PKK because "fighting has been between our men and soldiers inside Turkey, far from the Iranian border."

But Iran is bound by treaty with Turkey to fight the outlawed PKK, which has waged a 15-year insurgency against Ankara for self rule in Turkey's mainly Kurdish southeast.

In return, Turkey has pledged to fight the Iranian armed opposition group, the Iraq-based People's Mujahedeen.

Turkey says some 5,000 armed PKK militants have found refuge in northern Iraq since 1999, when the group declared a unilateral ceasefire after the capture of its leader, Abdullah Ocalan.

The truce was called off in June 2004.

"I warn Iran that their aggression against our party's positions in Iraq will have consequences," Judi said.

The details of casualties from Sunday's attack were not yet known.

Kurds make up the majority in three adjacent areas within Iraq, Iran and Turkey.

Tehran and Ankara have accused a number of separatist rebel groups of exploiting Kurdish-controlled areas in Iraq to launch attacks inside their countries.

For around a year, Iran has been battling border infiltrations by a Kurdish group called Pejak, which Tehran says is linked to the PKK.

Reports claim at least 120 Iranian police were killed and scores wounded in Kurdish rebel attacks last year, many of them blamed on Pejak.

Meanwhile, Turkey has massed troops along the border to intensify operations against PKK rebels who are sneaking into Turkey in growing numbers with the arrival of spring when snow melts and makes passage through the mountains easier.

On Sunday the Iranian newspaper Kayhan reported that four Iranian soldiers "were martyred in the Mahabad area while fighting with anti-revolutionary forces from the other side of the border."

Mahabad, situated close to Iran's border with both Turkey and Iraq, is an historic center of Kurdish nationalism.

Pejak confirmed reports of clashes with Iran saying in a statement on Sunday that four Iranian troops had been killed and four injured.

Turkey has long urged the United States and Iraq to root out the PKK from its bases in the mountains of northern Iraq, but it has been told that violence in other parts of the conflict-torn country was their priority.

US Secretary of State Condoleezza Rice called on Ankara during a visit last Tuesday to refrain from unilateral action against the Iraq-based Kurdish rebels, calling instead for renewed three-way cooperation to fight the threat.

The Kurdish conflict has claimed more than 37,000 lives since the PKK launched its separatist campaign in 1984.

Washington labels the PKK as a "terrorist" organization.