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The Islamic State group is regrouping in Iraq faster than in Syria, according to a new Pentagon report, underscoring the fluid nature of the security threat in the Middle East.

The assessment of the efforts by ISIS to reestablish a foothold in the region comes as the U.S. military is moving forward with a plan to pull out of Syria.

Citing the ongoing threat posed by the terror group, several lawmakers on both sides of the aisle and former defense officials have sharply criticized President Donald Trump's plan to draw down troops in Syria, as well as in Afghanistan.

The report, which was released Monday, confirms an NBC News story published last week that said a draft version had warned that ISIS could regain territory in six to 12 months in the absence of sustained military pressure.

An Iraqi forces member walks past a mural with the ISIS logo in the outskirts of Mosul in on March 1, 2017. Ahmad Al-Rubaye / AFP/Getty Images file

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"If Sunni socioeconomic, political and sectarian grievances are not adequately addressed by the national and local governments of Iraq and Syria, it is very likely that ISIS will have the opportunity to set conditions for future resurgence and territorial control," according to the Department of Defense Inspector General Quarterly Report about Operation Inherent Resolve.

"Currently, ISIS is regenerating key functions and capabilities more quickly in Iraq than in Syria, but absent sustained [counterterrorism] pressure, ISIS could likely resurge in Syria within six to 12 months and regain limited territory in the [middle Euphrates River valley]."

The report covers the three months from Oct. 1 to Dec. 31, 2018. Trump announced Dec. 19 that the U.S. military would be leaving Syria.

"We have defeated ISIS in Syria," he said via Twitter, "my only reason for being there during the Trump Presidency."

Monday's report describes ISIS as a "battle-hardened force" that attracts an estimated 50 new foreign fighters per month and still generates revenue in Syria through "oil smuggling operations, taxation, and criminal activities such as extortion and kidnapping in areas they do not control."

It says the group retains strength in rural swaths of Iraq but has devolved into a decentralized insurgency in Syria.

"ISIS' loss of territory (in Syria) has put increased pressure on its leadership, who have fewer places to hide, according to media reports," the report says. "As a result, more high-profile ISIS members have been captured or killed in recent months, although the seniormost members of the group have continued to elude death or capture."

In recent days, the number of U.S. troops in Syria has spiked to around 3,000 as more troops have moved in to help with the withdrawal, according to defense officials. Logistical support and security forces are in the country to help move out equipment and eventually troops.

The U.S. military remains under orders for a complete withdrawal in 120 days, ending the U.S. presence there in mid-spring. Roughly 5,000 U.S. troops are stationed in Iraq. In an interview that aired Sunday, Trump said it was vital to keep a military presence in the country to monitor Iran.

Iraqi President Barham Salih pushed back against the president Monday. "Those forces do not have the right to monitor many things, including watching Iran," Salih said. "We will not allow this."