Updated

Iranian President Hassan Rouhani said Saturday his country is ready to help Iraq if asked, adding that it has "no option but to confront terrorism," as hundreds of young Iraqi men poured into volunteer centers across Baghdad, answering a call by the country's top Shiite cleric to join the fight against Sunni militants advancing in the north.

Dozens climbed into the back of army trucks, chanting Shiite slogans and hoisting assault rifles, pledging to battle the Al Qaeda-linked Sunni group known as the Islamic State of Iraq and the Levant, (ISIS), which has launched a lightning advance across the country.

The massive response to the call by the Iranian-born Grand Ayatollah Ali al-Sistani, issued via his representative Friday, comes as sectarian tensions are threatening to push the country back toward civil war in the worst crisis since U.S. forces withdrew at the end of 2011.

"Given the current threat facing Iraq, defending the land, honor and holy places is a religious duty," Ayatollah Sistani said, according to The Wall Street Journal.

Speaking at a press conference Saturday, Rouhani suggested that the militants are linked to politicians who lost in parliamentary elections held in April.

"We will study if there is a demand for help from Iraq. Until today, no specific request for help has been demanded. But we are ready to help within international law," Rouhani said. "Entry of our forces (into Iraq) to carry out operations has not been raised so far. It's unlikely that such conditions will emerge."

Iran has built close political and economic ties with Iraq since the 2003 U.S.-led invasion toppled Saddam Hussein's Sunni-led government, and many influential Iraqi Shiites, including Prime Minister Nouri al-Maliki, have spent time in the Islamic Republic. Iran this week halted flights to Baghdad because of security concerns and said it was intensifying security on its borders.

After seizing Iraq's second city Mosul and Hussein's hometown of Tikrit earlier this week, the jihadist ISIS vowed to take the battle all the way to Baghdad and the Shiite heartland of southern Iraq, home to the faith's most revered shrines. Soldiers and policemen have melted away in the face of the lightning advance, and thousands have fled to the self-rule Kurdish region in northern Iraq.

In an address to military commanders in Samaara, al-Maliki warned that army deserters could face the death penalty if they don't report back to their units. But he insisted the crisis had a silver lining.

"This is our chance to clean and purge the army from these elements that only want to make gains from being in the army and police," he said. "They thought that this ist he beginning of the end but, in fact, we say that this is the beginning of their end and defeat."

On Saturday, insurgents seized the small town of Adeim in Diyala province after Iraqi security forces pulled out, said the municipal council, Mohammed Dhifan. Adeim is about 60 miles north of Baghdad. There was no official confirmation of the loss of the town.

Rouhani suggested the Islamic state could not have made such swift gains on its own, saying "other issues and coordination were involved." Figures from Hussein's deposed government as well as other Sunni militants are believed to have allied with the Islamic state in its campaign against the Shiite-led government in Baghdad.

"Those defeated (in elections) have resorted to bullets. This is a grave blunder," Rouhani said.

Also Saturday, the Iraqi government's counterterrorism department said the son of Saddam's vice president, Izzat Ibrahim al-Douri, was killed in an air raid by the Iraqi air force in Tikrit. It said Ahmed al-Douri was killed with some 50 other Saddam loyalists and ISIS fighters on Friday. The report could not be immediately verified.

The Associated Press contributed to this report.