Fears of a World War 3 have been sparked after the United States launched an attack that killed a top Iranian military leader, with experts warning that the strike could spark a region-wide war with Iran and its allies.

Late on Thursday, the Pentagon confirmed that President Donald Trump authorized a military strike at the Baghdad, Iraq, airport that killed Major-General Qassem Soleimani, who was seen as the second most powerful man in Iran. The killing sent shockwaves across the world, leading many to speak out against the attack and for Iran to vow retribution.

There were some immediate fears that the action could spark a larger war. The U.K.’s Express noted in its running update of events that “fears of World War 3 erupt after Trump orders brutal attack.”

While there was no indication that a world war could break out, some experts warned that the strike against one of the most revered military leaders in Iran could spiral into a larger conflict between Iran and its allies in the region. Richard N. Haass, president of the Council on Foreign Relations, warned that the conflict could expand well beyond Iran — possibly to the entire world.

Haass noted that a war on the horizon would be vastly different than recent U.S. military interventions in the Middle East.

“Make no mistake: any war with Iran will not look like the 1990 Gulf war or the 2003 Iraq wars,” Haass tweeted. “It will be fought throughout the region w a wide range of tools vs a wide range of civilian, economic, & military targets. The region (and possibly the world) will be the battlefield.”

A number of Trump’s opponents have also decried the attack, including former vice president and Democratic presidential candidate Joe Biden. While Biden denounced Soleimani and said that no American will mourn his death, he said that Trump’s actions were reckless and came without explanation to the American people about why the attack was carried out.

“President Trump just tossed a stick of dynamite into a tinderbox, and he owes the American people an explanation of the strategy and plan to keep safe our troops and embassy personnel, our people and our interests, bother here and abroad, and our partners throughout the region and beyond,” Biden said, via the Express.

Biden added that Iran would “surely respond” with force to the attack. It was not yet clear what kind of response Iran could be forming in the hours after the killing.