Retaliatory U.S. airstrikes against an Iranian-backed militia in Iraq and Syria have injected a volatile new element into the already tense relations between Washington and Tehran.

The strikes were in response to a Friday rocket attack on an Iraqi military base that wounded several U.S. troops and killed an American contractor — the first U.S. casualties after months of such shellings in Iraq.

On Monday, Iran warned of “consequences” for the U.S. strikes — the targeted militia group said 25 people were killed — but Iran hawks in Congress cheered the Pentagon's actions.

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Sen. Tom Cotton Tom Bryant CottonLoeffler calls for hearing in wake of Netflix's 'Cuties' Health care in the crosshairs with new Trump Supreme Court list Cruz says he wouldn't accept Supreme Court nomination MORE (R-Ark.), who on Saturday called for “swift and severe consequences” against Iran, called Sunday’s strikes by the U.S. a “justified response.”

“Tehran must stop attacking Americans or else face even graver consequences,” Cotton tweeted.

Sen. Lindsey Graham Lindsey Olin GrahamSenate Republicans signal openness to working with Biden Loeffler calls for hearing in wake of Netflix's 'Cuties' Quinnipiac poll shows Graham, Harrison tied in South Carolina Senate race MORE (R-S.C.) tweeted that he was “very glad” to see the U.S. response, adding that “the only language Iran understands is force.”

But others warned the situation could quickly spiral out of control.

“For months I have been deeply concerned about the seemingly increased risk of military escalation and tension in the region,” House Armed Services Committee Chairman Adam Smith David (Adam) Adam SmithWhen 'Buy American' and common sense collide Overnight Defense: Marine Corps brushes off criticism of Marines' appearance in GOP convention video | US troops injured in collision with Russian vehicle in Syria | Dems ask for probe of Vindman retaliation allegations Democrats press Pentagon watchdog to probe allegations of retaliation against Vindman brothers MORE (D-Wash.) said in a statement Sunday night condemning the Iraqi militia’s rocket attack.

“Now, more than ever, I cannot overstate the importance of increased diplomatic engagement with Iran to deter future aggression and slow the cycle of escalating violence that further destabilizes the region,” Smith added.

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The United States has about 5,000 troops in Iraq leading the international anti-ISIS coalition.

On Friday night, a U.S. defense contractor was killed and four service members were wounded in a barrage of more than 30 rockets targeting an Iraqi base in Kirkuk, according to the Pentagon.

That followed several similar attacks the United States has blamed on Iran-backed militias collectively known as Popular Mobilization Forces. A senior State Department official said Monday there have been 11 attacks on Iraqi bases housing coalition troops in the last two months.

The Iraqi government has worked to fold the Iran-backed militias into its security forces, but the United States views them as Iranian proxy forces.

Following early December attacks that injured Iraqi soldiers, Secretary of State Mike Pompeo Michael (Mike) Richard PompeoThe Hill's Morning Report - Sponsored by The Air Line Pilots Association - Pence lauds Harris as 'experienced debater'; Trump, Biden diverge over debate prep Trump's foreign policy successes confound his detractors It's time for a Jackson-Vanik Amendment for China MORE warned Iran that “any attacks by them, or their proxies of any identity, that harm Americans, our allies or our interests will be answered with a decisive U.S. response.”

In response to Friday’s attack in Kirkuk, the Pentagon said Sunday that it had struck three Kata'ib Hezbollah (KH) targets in Iraq and two KH targets in Syria. The targets included weapons depots and command-and-control locations the Pentagon said the militia used to plan its attacks.

“Iran and their KH proxy forces must cease their attacks on U.S. and coalition forces, and respect Iraq's sovereignty, to prevent additional defensive actions by U.S. forces,” Pentagon spokesman Jonathan Hoffman said in a statement.

In a brief statement to reporters at Mar-a-Lago alongside Pompeo and Joint Chiefs of Staff Chairman Gen. Mark Milley, Defense Secretary Mark Esper Mark EsperOvernight Defense: Trump hosts Israel, UAE, Bahrain for historic signing l Air Force reveals it secretly built and flew new fighter jet l Coronavirus creates delay in Pentagon research for alternative to 'forever chemicals' Oldest living US World War II veteran turns 111 Overnight Defense: US marks 19th anniversary of 9/11 attacks | Trump awards Medal of Honor to Army Ranger for hostage rescue mission | Bahrain, Israel normalizing diplomatic ties MORE called the retaliatory U.S. strikes “successful.”

The move was met with praise from other Iran hawks in the U.S., including former national security adviser John Bolton John BoltonThe Hill's Morning Report - Sponsored by The Air Line Pilots Association - Pence lauds Harris as 'experienced debater'; Trump, Biden diverge over debate prep DOJ launches probe into Bolton book for possible classified information disclosures Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE, who has been critical of the Trump administration in other policy areas since leaving in September. Bolton called the strikes “overdue” and “a good first step.”

“The ayatollahs must pay a steep price for their ongoing support for terrorism,” Bolton tweeted.

Sen. Marco Rubio Marco Antonio RubioHillicon Valley: DOJ indicts Chinese, Malaysian hackers accused of targeting over 100 organizations | GOP senators raise concerns over Oracle-TikTok deal | QAnon awareness jumps in new poll Intelligence chief says Congress will get some in-person election security briefings Republican Senators raise concerns over Oracle-TikTok deal MORE (R-Fla.) accused Iran of “playing a very dangerous game,” warning in a tweet that there is a “tense situation emerging in #Iraq with serious potential to escalate into a broader & dangerous conflict.”

The tit-for-tat attacks in Iraq come amid a faceoff between Washington and Tehran that started last year when President Trump Donald John TrumpBarr criticizes DOJ in speech declaring all agency power 'is invested in the attorney general' Military leaders asked about using heat ray on protesters outside White House: report Powell warns failure to reach COVID-19 deal could 'scar and damage' economy MORE withdrew from the Obama-era nuclear deal with Iran. Trump significantly tightened sanctions as a result, leading to Iran breaching key limits of the deal one by one.

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The United States has also blamed Iran for several attacks on oil tankers in the Gulf region, accusations that Iran has denied. Tensions reached a fever pitch over the summer when Iran shot down a U.S. drone, leading Trump to come within minutes of launching a retaliatory strike on Iran.

Amid increased tensions, the United States has deployed an additional 14,000 U.S. troops to the Middle East since the summer.

With that backdrop, some are now warning of the potential for the weekend strikes to lead to greater conflict, particularly amid what critics describe as Trump’s inconsistent approach to the Middle East.

"I have concerns as to how the president — what the president’s policy is in regard to Iraq and Syria?" Sen. Ben Cardin Benjamin (Ben) Louis CardinPPP application window closes after coronavirus talks deadlock Congress eyes tighter restrictions on next round of small business help Senate passes extension of application deadline for PPP small-business loans MORE (D-Md.) said Monday on CNN. "Clearly, Iran is a dangerous actor. They do things that are extremely against, very much against our national security interests. We know that. They’re acting in a way that destabilizes that region of the world. Our problem is, what is the president's — what is U.S. policy?"

Benjamin Friedman, policy director at the libertarian-leaning Defense Priorities, said in a statement Sunday that “retaliatory strikes risk escalation to a broader regional war that would harm U.S. interests.”

On Monday, Pompeo stressed the retaliatory strikes were a “defensive action.”

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“President Trump’s been pretty darn patient, and he’s made clear at the same time that when Americans’ lives were at risk we would respond, and that’s what the Department of Defense did yesterday,” Pompeo said on “Fox & Friends.”

A senior State Department official similarly stressed to reporters Monday the “defensive” intent of the strikes but added that they were also “aimed at deterring Iran.”

“We are not looking for any conflict in the Middle East. These were defensive strikes. But we are not going to let Iran get away with using a proxy force to attack American interests,” the official said.

Kata'ib Hezbollah warned Monday of retaliation.

“Our battle with America and its mercenaries is now open to all possibilities,” the militia group said in a statement, according to The Associated Press. “We have no alternative today other than confrontation and there is nothing that will prevent us from responding to this crime.”

Meanwhile, in a statement carried by the state-run Islamic Republic News Agency, Iranian Foreign Ministry spokesman Abbas Mousavi said the United States has “openly shown its support to terrorism and shown its negligence to the independence and national sovereignty of countries.”

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"It must accept responsibility of the consequences of the illegal attacks,” Mousavi added.

There’s also the potential that the recent strikes will prompt the Iraqi government to kick U.S. troops out of the country. Iraq has been rocked by anti-government protests in recent months, in part over concerns about Iran’s influence in the country. But there have also been frustrations in recent years over all foreign influence.

Iraq’s outgoing prime minister told Esper in a phone call Sunday not to carry out the strikes, the prime minister’s spokesman told the Iraqi News Agency.

In a strongly worded statement Monday, the Iraqi government warned that the U.S. strikes endangered the “security and sovereignty” of its country by acting unilaterally without approval from Iraqi authorities. The government added it would “review the relationship” with the U.S.-led coalition in Iraq, saying the attack “violates the goals and principles” of the international anti-ISIS coalition.

Asked about Iraq’s response Monday, a senior State Department brushed off threats of consequences, telling reporters that “we don’t have any fears in this regard.”

“We have warned the Iraqi government many times, and we’ve shared information with them, to try to work with them to carry out their responsibility to protect us as their invited guests,” the official added. “It’s their responsibility and duty to protect us, and they have not taken the appropriate steps to do so.”