The world braced on Sunday for Iran's response after the U.S. attacked key Iranian nuclear sites, joining Israel in the biggest Western military action against the Islamic Republic since its 1979 revolution.

Iran vowed to defend itself a day after the U.S. dropped 30,000-pound bunker-buster bombs onto the mountain above Iran's Fordow nuclear site while American leaders urged Tehran to stand down and pockets of anti-war protesters emerged in U.S. cities.

In a post to the Truth Social platform on Sunday, U.S. President Donald Trump raised the idea of regime change in Iran. "It’s not politically correct to use the term, 'Regime Change,' but if the current Iranian Regime is unable to MAKE IRAN GREAT AGAIN, why wouldn’t there be a Regime change??? MIGA!!!" he wrote.

Iran and Israel continued to trade volleys of missile attacks. An Israeli military spokesperson said Israeli fighter jets had struck military targets in western Iran. Earlier, Iran fired missiles that wounded scores of people and flattened buildings in Tel Aviv.

The U.S. Department of Homeland Security warned of a "heightened threat environment" in America, citing the possibility of cyber attacks or targeted violence. Law enforcement in major U.S. cities stepped up patrols with a focus on religious, cultural and diplomatic sites.

The U.S. State Department issued a security alert for all U.S. citizens abroad that warned of the potential for demonstrations against Americans and travel disruptions due to closed airspace across the Middle East, calling on them to "exercise increased caution."

Tehran has so far not followed through on its threats of retaliation against the United States - either by targeting U.S. bases or trying to choke off global oil supplies - but that may not hold.

Speaking in Istanbul, Iranian Foreign Minister Abbas Araqchi said his country would consider all possible responses. There would be no return to diplomacy until it had retaliated, he said.

"The U.S. showed they have no respect for international law. They only understand the language of threat and force," he said.

Trump, in a televised address, called the strikes "a spectacular military success" and boasted that Iran's key nuclear enrichment facilities had been "completely and totally obliterated."