Iran declared the Strait of Hormuz closed on Monday and threatened to attack any vessels attempting to pass through the strategic chokepoint, as the US military said six American service members had been killed in the escalating conflict with Iran.

Ebrahim Jabari, a senior adviser to the commander-in-chief of Iran's Islamic Revolutionary Guard Corps (IRGC), told state television that the waterway was closed and that naval forces would set ablaze any ship trying to transit it. "We will not let oil be exported from the region," he added, according to Iranian media.

The remarks came as shipping firms, including Maersk and MSC, have already halted transit through the narrow strait – which carries about one-fifth of global oil and LNG supply – amid soaring insurance costs and Iranian warnings.

US Central Command said the American death toll rose to six after the recovery of remains from a facility struck during Iran's initial retaliatory attacks, including a hit on a makeshift operations center at Kuwait's Shuaiba port. The deaths mark the first US service members killed in action in the operation that began on Saturday.

It also said three US fighter jets were mistakenly shot down by Kuwait air military.

Rallies were held across the US protesting President Donald Trump's military operation against Iran, with large crowds condemning the strikes and many demonstrators holding signs and chanting slogans such as "Hands Off Iran" and "Stop the War on Iran."

A CNN poll released on Monday showed 59% of Americans disapprove of the US strikes on Iran. 

The developments follow US and Israeli strikes on Iran that killed Supreme Leader Ali Khamenei and senior officials, prompting Iranian missile and drone responses targeting Israeli territory and US military assets across the Middle East.

The Iranian Red Crescent said at least 555 people have been killed in Iran. At least 11 people were killed in strikes in Israel.

Iran's UN envoy Amir Saeid Iravani on Monday urged the UN Security Council to condemn the attacks as aggression and war crimes, while Trump said he would not rule out sending ground troops and estimated the campaign could last four to five weeks.

The conflict widened into Lebanon on Monday after Hezbollah launched rockets and drones toward northern Israel, including a barrage targeting a missile defense site south of Haifa, in retaliation for Khamen's killing and repeated Israeli strikes since the November 2024 ceasefire.

Israeli forces responded with heavy air and naval strikes on Hezbollah positions in Beirut's southern suburbs, southern Lebanon and other areas, killing at least 52 people and wounding scores more, Lebanese authorities said.