Rockets struck Baghdad's international airport compound Thursday, as the country tries to contain anti-government protests which have shaken the foundation of the Iraqi government. The attack appears to be the latest in what a senior U.S. military official described as a dangerously escalating campaign by Iran-backed militias.

The protests, which have swept cities from Baghdad to Basra over the last two-and-a-half months, have laid bare the Iraqi government's limited ability to control Iran-backed paramilitary forces that are now part of the country's official security forces. And they have raised fears that tensions among the largest of roughly 30 different Iran-backed groups will divert attention from fighting a resurgent ISIS.

More than 450 protesters have been killed since the anti-government protests began Oct. 1 — many of the deaths blamed on Iran-backed militia forces. Demonstrators have demanded an end to what they consider an Iranian stranglehold on Iraq's security, politics and economy — posing the biggest threat to Iranian influence in Iraq since 2003. Although the demonstrations have been largely peaceful, in the holy cities of Najaf and Karbala, protesters set fire to the Iranian consulates, along with a shrine tied to an Iran-backed political figure.

Demonstrators have also demanded a new Iraqi government. Iraq's prime minister recently resigned in response to the protests and the Iraqi parliament is under pressure to pass reforms to limit the influence of traditional political parties.

The rockets fired at the airport Thursday caused no casualties or serious damage, but they followed a string of attacks over the past five weeks that U.S. officials say have escalated both in frequency and potential lethality. On Monday, rockets fired near a State Department-run diplomatic security compound in the airport compound wounded at least five Iraqi counterterrorism forces, one of them seriously.

"I think that attack is part of a broader trend that is attacking the U.S. and coalition presence in Iraq, as opposed to the episodic harassing fire that we've become used to," a senior U.S. military official told NPR in Baghdad. He spoke on condition of anonymity to be able to speak more candidly about the threats.

The official cited a Nov. 8 attack on U.S. forces at the Qayyarah West Airfield base, south of Mosul, in which 31 rockets were launched from the back of a specially modified flatbed truck. That attack, he said, used 107-mm rockets, most of which landed off the compound with no reported injuries.

The next major attack, on Dec. 3, used a similarly adapted truck to remotely launch larger 122-mm rockets at al-Asad base in western Anbar province, according to multiple military officials. The U.S. military official said some of the rockets landed near a dining facility — vacant at the time. Al-Asad is the base where Vice President Mike Pence visited American forces in November, as did President Trump last Christmas.

"That's a bit different than one or two rockets that are harassing fire," said the U.S. military official, referring to rockets regularly launched into Baghdad's Green Zone, where the American Embassy is located. Those rockets normally land in empty fields or fall short and fall into the Tigris River.

The official says even larger 240 mm rockets — which he says the U.S. knows to be Iranian-made — were used in Monday's attack near the State Department-run compound.

The U.S. military official says intercepted communications show clear links between the attacks on military bases and Kataib Hezbollah, the largest of the Iranian-backed paramilitaries in Iraq. The group is part of the Popular Mobilization Forces formed to fight ISIS in 2014 and is nominally integrated into Iraq's official security forces.

"We know it's KH and [the Iraqi government] know[s] it's KH. Why is someone not holding them to account?" asked the U.S. military official. "Now you have one element of supposedly, purportedly legitimate security forces causing casualties on another element of the Iraqi legitimate security forces — and threatening a coalition that's here to defeat ISIS at the invitation of the Iraqi government."

A Kataib Hezbollah spokesman, asked about the allegations, told NPR he was not yet authorized to comment on them.

The Iraqi government has said it is investigating the Dec. 9 attack which wounded members of its counterterrorism forces. Up to now, it has been consistently unwilling to publicly blame militias that are allegedly under the prime minister's command. It has placed blame on "unknown groups" or unspecified "third parties" for many protester deaths.

U.S. military personnel in Iraq were placed on heightened alert after Thursday's attack. The U.S. Embassy earlier this year evacuated hundreds of staff in response to what it said were increased Iranian threats. The U.S. government previously closed down its consulate in Basra, following a rocket attack on the edge of the compound where it was based.

The closure has left the United States with no diplomatic presence south of Baghdad. The U.S. military, limited to anti-ISIS operations and training, also has no presence in Iraq's southern provinces, which are largely controlled by Iranian-backed militias.

The uptick in attacks attributed to Iran-backed groups has raised fears of a U.S. military response among many Iraqis who are worried their country is caught in the middle of a U.S.-Iranian conflict. Such a response would almost certainly spark retaliation by Iran and could further destabilize Iraq's weak caretaker government.

U.S. officials have long described the death or injury of a American citizen as a red line that would spark retaliation.

"My fear is if [the Iraqi government] is not willing to take action, and if they're not willing to tamp this down, then we're going to get to a point where we are going to be backed into a corner," says the U.S. military official. "We won't just eat rockets all day until a couple of us are killed."

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