Iran continues to expand its influence in Iraq through at least 80,000 Shiite militia fighters, prompting concerns that the Shiite country may replace the Islamic State (ISIS/ISIL) should it be defeated, reports Fox News.

Col. Chris Garver, a Baghdad-based U.S. military spokesman, confirmed to Fox News that the overall number of Shiite fighters, including anti-American troops backed by Iran, has grown to 100,000.

However, he indicated that “not all the Shia militias in Iraq are backed by Iran,” adding that “the [Iranian-backed] Shia militia are usually identified at around 80,000.”

The 100,00 figure was first floated in late July by the Tampa Bay Times, which cited Army Gen. Joe Votel, the U.S. Central Command (CENTCOM) chief, as its source.

Gen. Votel indicated that there are 100,000 Shiite militia fighters in Iraq, adding that “many” are backed by Iran.

The Shiite militias in Iraq are collectively known as al Hashed al Shabi, or Popular Mobilization Forces (PMF).

Citing estimates from the Iraqi government and the PMF itself, the Associated Press (AP) reported in March that “the more than 50 Shiite militias in Iraq” at the time had at least 60,000 fighters, adding that “they are backed by tanks and weapons, and have their own intelligence agency, operations rooms and court of law.”

Fox News now reports:

The ranks have swelled inside a network of Shiite militias known as the Popular Mobilization Forces. Since the rise of Sunni-dominated ISIS fighters inside Iraq more than two years ago, the Shiite forces have grown to 100,000 fighters, Col. Chris Garver, a Baghdad-based U.S. military spokesman, confirmed in an email to Fox News. The fighters are mostly Iraqis…

Whether the force size is 80,000 or 100,000, the figures are the first-known estimates of the Iranian-backed fighters. The figure first surfaced in a recent Tampa Bay Times article and marks the latest evidence of Tehran’s deepening involvement in the war against ISIS, with the U.S. military also confirming that Russian bombers are now flying into Syria from a base in Iran. The growth also could create greater risk for Americans operating in the country, as at least one Iran-backed group vowed earlier this year to attack U.S. forces supporting the Iraqis.

In March, an Iranian-backed militia, identified as one of the deadliest in Iraq, threatened U.S. troops. The threat came as the Shiite-led Iraqi government expressed concern over the growing influence of Shiite fighters in their country.

Qassem Soleimani, an Iranian general who commands the Islamic Revolutionary Guard’s Quds Force and has been linked to the death of nearly 500 Americans, is now on the ground in Iraq ahead of an upcoming offensive to retake Mosul, Iraq’s second-largest city, reports the Long War Journal.

Mosul is believed to be ISIS’s last major stronghold in Iraq.

AP noted in March:

The larger [Shiite] militias [in Iraq], like Asaib Ahl al-Haq, the Hezbollah Brigades, Badr and the Peace Brigades, have been in place since soon after the 2003 ouster of Saddam Hussein. They are linked to political parties, effectively forming armed branches for politicians.

But the ranks of the militias swelled dramatically after [the Islamic State] overran nearly a third of Iraq in the summer of 2014 and Grand Ayatollah Ali al-Sistani, Iraq’s top Shiite cleric, called on able-bodied males to fight IS [Islamic State]. At the time, tens of thousands turned out.

The Iran-backed Shiite militia that threatened U.S. Marines deployed to Iraq was identified as the Asaib Ahl al-Haq (League of the Righteous), which operates under Gen. Soleimani and has been linked to more than 6,000 attacks against American troops, according to the Institute for the Study of War.

Although Shiite militias have participated in some U.S.-backed operations in Iraq, Col. Garver emphasized during a press briefing Tuesday that the United States is “not coordinating” with the Iranians.

He said:

We are not coordinating with the Iranians in any way. We’re not working with them in any way… However, the government of Iraq comes up with a plan… for the seizure of Mosul.

Russia has been fighting on behalf on dictator Bashar al-Assad in Syria along with Iranian-backed Shiite groups, including the Lebanese terror organization Hezbollah.

Fox News notes:

The threat to American troops remains. Last month, firebrand Shia cleric Muqtada al-Sadr — responsible for attacks against U.S. troops in Iraq a decade ago – once again called for his supporters to kill American troops…

In addition to the up to 100,000 Iranian-backed forces in Iraq, there are thousands of Iranian-backed forces in Syria as well in support of President Bashar al-Assad. Some of these Iranian-backed forces come from as far as Afghanistan and hundreds have recently died fighting Syrian rebels in the city of Aleppo, according to recent reports.