President Hassan Rouhani says Iran would do whatever it takes to protect revered Shia Muslim holy sites in Iraq against ISIL terrorists fighting the Baghdad government.

"Dear Karbala, Dear Najaf, Dear Kadhimiyah and Dear Samarra, we warn the great powers and their lackeys and the terrorists, the great Iranian people will do everything to protect them," he said, naming the sites of the shrines in an emotive speech in Khoramabad, near the Iraq border on Wednesday.

Rouhani on Saturday pledged to help the Iraqi Prime Minister Nuri al-Maliki's government if it asked for assistance, though at that time no such request had been forthcoming.

In his speech, Rouhani mentioned petitions signed by Iranians who said they were willing to fight in Iraq "to destroy the terrorists and protect the holy sites", which are visited by hundreds of thousands of Iranian pilgrims annually.

"Thank God there are enough volunteers Shias, Sunnis and Kurds in Iraq to fight the terrorists," he added.

The Iranian pledges follow a call by top Shiite cleric Grand Ayatollah Ali al-Sistani for Iraqis to volunteer to resist the onslaught spearheaded by the so-called Islamic State of Iraq and the Levant (ISIL), who hold the major cities of Mosul and Tikrit and are fighting north of the capital.

The major Shia shrines in Iraq are in Najaf and Karbala, south of the capital, in the district of Kadhimiyah in Baghdad and in Samarra to the north, which the militants have made repeated, but so far unsuccessful, efforts to enter.

At least 5,000 Iranians have pledged online to defend Iraq's Shia shrines against the ISIL terrorists, a conservative news website in Iran reported on Tuesday.