US President Donald Trump on Monday said the U.S. will impose fees in the Strait of Hormuz “at the rate of 20% on all cargo shipped,” after declaring America the “guardian” of the major oil shipping route.

Axar.az reports, citing CNBC, that oil prices jumped, and stock indexes fell, following Trump’s announcement.

The strait, which saw 20% of the world’s oil trade before being choked off at the start of the war in late February, “is OPEN, and will remain OPEN, with or without Iran,” Trump wrote in the post.

All countries other than Iran will “have fair and open use of the Strait,” Trump wrote. The assertion came amid new exchanges of fire between the U.S. and Iran that have put the prospect of a peace deal further out of reach, while once again slowing tanker traffic in the strait to a trickle.

“The U.S.A. will be, from this point forward, known as ‘THE GUARDIAN OF THE HORMUZ STRAIT,’” he claimed. “But as such, and as a matter of FAIRNESS, will be reimbursed, at the rate of 20% on all cargo shipped, for any and all costs necessary to do the job of providing safety and security to this very volatile section of the World.”

“The process and formation will begin immediately,” he added.