U.S. forces launched a new wave of military strikes against Iran late Tuesday, officials said, citing a desire to impose costs on Tehran following Iranian attacks on commercial shipping.

U.S. Central Command, which oversees military operations in the Middle East, said in a statement issued shortly after 5 p.m. that it had begun “powerful strikes” in response to attacks on vessels that had been “crewed by innocent civilians” in the Strait of Hormuz. The vital waterway has been at the center of tensions between Washington and Tehran for months.

U.S. officials called the Iranian attacks “unwarranted, dangerous, and a clear violation” of a ceasefire to which U.S. and Iranian officials agreed. The truce has been shaky from the start, with strikes launched by both sides.

A U.S. official, speaking on the condition of anonymity because of the sensitivity of the issue, said he anticipates that Central Command will say more about the strikes after they are complete. They are expected to go on for some time.

The U.S. strikes were launched hours after the Trump administration revoked a waiver for Iranian oil sales that had been put in place as part of the agreement struck between the United States and Iran last month — the primary benefit to Iran of the ceasefire deal.

The reversal immediately pushed up the cost of Brent crude, the international benchmark for oil prices.