The U.S. military said on Saturday that it struck three Iranian oil tankers after Navy warships were targeted with missiles, and warned that it would "if necessary, destroy Iran's limited and exposed oil fleet."

The strikes came a day after President Donald Trump sought to minimize the conflict as "small potatoes," and they sustain a renewed tilt back toward fighting after six months of on-again, off-again war that began with U.S. and Israeli attacks on Feb. 28. Both sides have sought to inflict military and economic pain on the other, and negotiations have collapsed.

According to the military's statement, a U.S. aircraft carrier and a destroyer evaded "multiple unprovoked Iranian attacks" while patrolling in the region, and no U.S. personnel were hurt. Two Iranian oil carriers were "permanently disabled," while a third, unladen tanker was destroyed.

The U.S. statement said the tankers were part of a shadow network helping to fund Iran's powerful Revolutionary Guard and its armed proxies in the region. Such networks have long been used to move Iranian crude in defiance of international sanctions, which have tightened repeatedly over Iran's nuclear program and its support for regional armed groups.

Earlier, Iranian state TV reported that four U.S. missiles struck a tanker about six miles (10 kilometers) from Kharg Island, home to a terminal through which Iran exports most of its oil. Kharg Island has been repeatedly targeted during the war, including U.S. strikes on military sites there in March.

The U.S. said one tanker was struck off Kharg Island and another near Jask, east of the Strait of Hormuz. The unladen tanker was hit in the Gulf of Oman. The statement shared what it described as video footage of the strikes.

"We will not hesitate to defend American forces, and if necessary, destroy Iran's limited and exposed oil fleet," said Adm. Brad Cooper, the head of U.S. Central Command.

The strikes came nearly a week after the U.S. and Iran resumed attacks following a month of relative calm, with the strait and Iranian communities along it again being targeted. At least five people were killed earlier in the week during a U.S. bombardment of southern Iran. One strike hit a wedding.

The resumption of fighting followed new U.S. efforts to apply economic pressure on Tehran, whose hard-line new senior leaders have signaled a willingness to dig in after weathering decades of sanctions.

Meanwhile, the issue that helped lead to the war — Iran's nuclear program — was meant to be addressed in negotiations that fell apart soon after the U.S. and Iran signed a memorandum of understanding in mid-June. Instead, Tehran's new leverage focuses on the strait, which is crucial to global oil and natural gas shipments and was seen as an international waterway before the war began. Before the conflict, roughly a fifth of the world's traded oil passed through the Strait of Hormuz, making any sustained disruption there a global energy concern.

The U.S. military has been helping to guide ships through the strait as Tehran asserts control and targets some vessels, but overall traffic remains low. How long shipping stays depressed — and whether Washington acts on its warning against Iran's tanker fleet — will shape the conflict's next phase.

This story was originally featured on Fortune.com.