The U.S. and Iran have a deal to reopen the Strait of Hormuz, but the world's largest tanker owner is warning that shipping companies aren't going to just flip a switch and start sailing through again.

According to a report by the Financial Times on Tuesday, Jotaro Tamura, head of Mitsui OSK Lines, said operators want to see proof that the agreement is actually working on the water before they send their vessels through the route. The strait handles more than a fifth of the world's crude oil supply, so the stakes are high.

"A simple agreement" won't be enough to reassure operators, Tamura said, noting that earlier attempts to reopen the route didn't pan out. "I think it's reasonable to assume that it may take at least a couple of weeks" for traffic to return to normal, he added.

Tamura made those comments before President Trump announced the deal, but the company later confirmed his view hasn't changed.

Philip Belcher, marine director at Intertanko, urged restraint in the report, noting that some vessels have been trying to sneak through under cover of darkness with their GPS switched off. About 135 ships passed through the route daily before the conflict, so getting back to that level won't happen overnight.

Tamura also pushed back on Iran's idea of imposing a transit fee, arguing it would violate international rules on freedom of navigation. Mitsui OSK Lines managed to move four vessels out of the Gulf before the reopening deal, and Tamura said the company didn't pay any fees to Iran. The company still has at least seven ships waiting to pass through the strait.

On the political side, Vice President JD Vance said Iran could be allowed access to a $300 billion reconstruction fund if it sticks to the agreement. But that access is contingent on Tehran's compliance — no free money without following through.

For now, the deal is signed, but the real test is whether ships will actually start moving. And that might take a while.