Pakistani Shiite Muslims demonstrate over the U.S. airstrike in Iraq that killed Iranian Revolutionary Guard Gen. Qassem Soleimani, in Islamabad, Pakistan, Friday, Jan. 3, 2020. Iran has vowed "harsh retaliation" for the U.S. airstrike near Baghdad's airport that killed Tehran's top general and the architect of its interventions across the Middle East, as tensions soared in the wake of the targeted killing. Credit: B.K. Bangash | AP

BAGHDAD — Iran vowed “harsh retaliation” for a U.S. airstrike near Baghdad’s airport that killed a top Iranian general who had been the architect of its interventions across the Middle East as Maine’s congressional delegation noted soaring tensions in the wake of the killing.

The killing of Qassem Soleimani, the head of Iran’s elite Quds Force, marks a major escalation in the standoff between Washington and Iran, which has careened from one crisis to another since President Donald Trump withdrew from the 2015 nuclear deal and imposed crippling sanctions.

Soleimani “made the death of innocent people his sick passion,” Trump said from his estate in Palm Beach, Florida, adding “a lot of lives would have been saved” if he’d been hunted down years ago.

“We take comfort in knowing that his reign of terror is over,” he said.

The United States urged American citizens to leave Iraq “immediately” following the Friday airstrike at Baghdad’s international airport that Iran’s state TV said killed Soleimani and nine others. The embassy in Baghdad, which was attacked by Iran-backed militiamen and their supporters earlier this week, is closed.

Around 5,200 American troops are based in Iraq to train Iraqi forces and help in the fight against Islamic State group militants. U.S. embassies also issued a security alert for Americans in Lebanon, Bahrain Kuwait and Nigeria.

Iran’s Supreme Leader Ayatollah Ali Khamenei warned that “harsh retaliation is waiting” for the U.S. after the airstrike, calling Soleimani the “international face of resistance.” Khamenei declared three days of public mourning. Iranian President Hassan Rouhani called the killing a “heinous crime” and vowed his country would “take revenge.”

Thousands of worshippers in the Iranian capital Tehran took to the streets after Friday Muslim prayers to condemn the killing, waving posters of Soleimani and chanting, “Death to deceitful America.”

The targeted strike, and any retaliation by Iran, could ignite a conflict that engulfs the whole region, endangering U.S. troops in Iraq, Syria and beyond. However, the attack may act as a deterrent for Iran and its allies to delay or restrain any potential response. Oil prices surged on news of the airstrike and markets were mixed.

The killing promised to further strain relations with Iraq’s government, which is allied with both Washington and Tehran and has been deeply worried about becoming a battleground in their rivalry. Iraqi politicians close to Iran called for the country to order U.S. forces out.

The Pentagon said it killed the 62-year-old Soleimani because he “was actively developing plans to attack American diplomats and service members in Iraq and throughout the region.” It accused Soleimani of approving orchestrated violent protests at the U.S. Embassy in Baghdad.

The strike, on an access road near Baghdad’s airport, was carried out Friday by an American drone, according to a U.S. official. Soleimani had just disembarked from a plane arriving from either Syria or Lebanon, a senior Iraqi security official said.

The attack comes at the start of a year in which Trump faces both a Senate trial following his impeachment by Congress and a re-election campaign. It marks a potential turning point in the Middle East and represents a drastic change for American policy toward Iran.

The tensions are rooted in Trump’s decision in May 2018 to withdraw the U.S. from Iran’s nuclear deal with world powers, struck under his predecessor, Barack Obama. Since then, Tehran shot down a U.S. military surveillance drone and seized oil tankers.

The U.S. also blames Iran for other attacks targeting tankers and a September assault on Saudi Arabia’s oil industry that temporarily halved its production. Supporters of Friday’s strike said it restored U.S. deterrence power against Iran. Trump allies were quick to praise the action.

Others, including Democratic White House hopefuls, criticized Trump’s order. Democratic presidential candidate Joe Biden said Trump had “tossed a stick of dynamite into a tinderbox,” saying it could leave the U.S. “on the brink of a major conflict across the Middle East.”

In statements, the four members of Maine’s congressional delegation were careful to label Soleimani as a terrorist or a threat to American interests while saying the move would risk escalating tensions between the U.S. and both Iran and Iraq.

Both of Maine’s U.S. senators, Republican Susan Collins and independent Angus King, voted for a defeated amendment in June that would have barred unauthorized strikes against Iran with some exceptions. Collins said in a statement she was briefed by Vice President Mike Pence, who assured her the U.S. would “defend Americans in Iraq and around the world.”

She said the strike “does risk further military escalation” and Congress should not be sidelined. King said he was concerned with “violent reprisals” from Iran in the short term and “dangerous and bloody consequences” in the long term.

Rep. Jared Golden, a Democrat from the 2nd District and a Marine veterans of the Iraq and Afghanistan wars, said the strike raised “important questions” about Trump’s goals and strategy in the Middle East after pledging to stop “endless wars” in the region. Rep. Chellie Pingree, a Democrat from Maine’s 1st District, said the move “deliberately escalates conflict with Iran.”

BDN writer Michael Shepherd and Associated Press writers Robert Burns, Zeke Miller, Lolita C. Baldor, Jon Gambrell, Aya Batrawy, Nasser Karimi, Amir Vahdat, Bassem Mroue, Sarah El Deeb, Joseph Krauss and Josef Federman contributed to this report.