Iran fired multiple attack drones toward the Strait of Hormuz on Friday, and the United States shot down at least four of them, U.S. Central Command said, in the latest spasm of violence to threaten the fragile cease-fire between the two countries.

Central Command said on social media that the one-way attack drones “posed an immediate threat to regional maritime traffic.” U.S. forces subsequently struck Iranian coastal surveillance radar sites in Goruk and on Qeshm Island, off the coast of Iran, to thwart more attacks, the military said.

“American forces remain vigilant and postured to respond to unjustified Iranian aggression in self-defense,” Central Command said.

American forces in the past month have helped coordinate the passage of more than 100 commercial vessels through the strait — into and out of the Persian Gulf — a U.S. official said on Friday, even as travel through the waterway remains risky amid stalled negotiations to end the war. The official spoke on condition of anonymity to discuss operational matters.

The tit-for-tat military volleys late Friday risked rekindling tensions in the region just two days after one civilian was killed and dozens of others were injured at Kuwait’s international airport, after Iran fired a barrage of missiles and drones at the country. It was one of the most significant Iranian strikes and the first known fatality in a Gulf nation since the cease-fire took effect in early April.

Since the beginning of the war, Iran has also struck major energy facilities, ports and hotels in Gulf countries as it seeks to punish U.S. allies and increase economic pressure on the Trump administration. The attack on Wednesday involved 13 ballistic missiles and 17 drones, Kuwait said.

U.S. Central Command said on Wednesday that Iran had also fired missiles and drones at Bahrain, another U.S. ally in the Gulf, as well as at civilian sailors in nearby waters. None of those strikes hit their targets, the military said.

Although U.S. and Iranian attacks have slowed significantly since the two sides announced a cease-fire, they have not stopped, with both sides declaring they are defending themselves.

In its statement on Wednesday, for instance, U.S. Central Command said that it had conducted “self-defense strikes” on an Iranian military ground control station on Qeshm Island.