GEOFF BENNETT: Today, the White House said American and Iranian negotiators continue to speak, even as both sides draw a hard line in public.

The U.S.

military is launching strikes on Iran again tonight, while President Trump has suggested he will soon escalate even further.

For its part, Iran today said control of the Strait of Hormuz is a red line.

Nick Schifrin speaks now to the head of the U.N.

's Maritime Organization about the U.S.-Iran struggle for the strait.

NICK SCHIFRIN: Today, the waterway that is supposed to supply one-fifth of the world's energy is almost entirely shut.

And, today, Iran released video of what it called attacks on U.S.

allies and vowed to never give up control of the Strait of Hormuz.

EBRAHIM ZOLFAGHARI, Iranian Military Spokesperson (through translator): Under no circumstances and in no way will we allow the United States as a foreign and extraregional country to interfere in the Strait of Hormuz.

This is Iran's inviolable red line.

ARSENIO DOMINGUEZ, Secretary-General, International Maritime Organization: The longer and the more that shipping gets used and weaponized, the more that everybody around the world is going to be affected.

NICK SCHIFRIN: Arsenio Dominguez is the secretary-general of the International Maritime Organization, the U.N.

's body in charge of the safety and security of international shipping.

We saw shipping data today that indicated that the number of ships leaving the Strait of Hormuz was in the single digits.

That's a reduction even from the last few days.

Can you confirm that?

And why?

ARSENIO DOMINGUEZ: It is correct.

And, of course, that also responds to the call that I have been making for ship owners and ship operators not to put at risk the lives of innocent seafarers and to try to transit across the Strait of Hormuz, particularly because we have seen an escalation in the conflict between the two parties in the last few days.

And we're going back to where we were before at the beginning of the conflict.

NICK SCHIFRIN: From your perspective, how important is it that full -scale war not resume?

ARSENIO DOMINGUEZ: It's very important.

The impact on the global economy, the shortages of fuel, LNG, as well as fertilizers for food security, these are the things that we're seeing where the costs are going up and the shortages are happening.

So, for the longer this conflict goes on, the more difficult that it will be for shipping to supply that over 80 percent of global goods around the world, and, of course, for the innocent seafarers to be able to go back to the normal lives.

NICK SCHIFRIN: U.S.

strikes are designed to prevent Iran from attacking ships.

KAROLINE LEAVITT, White House Press Secretary: The reason for the recent strikes over the course of the last several days is because Iran violated the memorandum of understanding that we struck with them.

Specifically, in the memorandum of understanding that they signed, they were not to fire on commercial vessels moving through the Strait of Hormuz.

NICK SCHIFRIN: But Iran keeps striking unless ships sail on an Iranian-approved route.

There have been various attempts by the IMO and Oman, as well as by the U.S.

military, to try and create a passageway that would hug the Omani coast, rather than the Iranian coast, so that ships could transverse safely.

Why hasn't that worked?

ARSENIO DOMINGUEZ: Right now, the main reason is the conflict.

Until we have guarantees that no vessel from any country will be affected or attacked, we cannot resume transit in the Strait of Hormuz right now.

This is why the negotiations are so important.

NICK SCHIFRIN: And, today, there is a new risk to global shipping.

In Yemen, the Iran-backed Houthis, who've previously seized and attacked ships, are now threatening to try and close the Bab al-Mandab, the gateway to the Red Sea, 1,300 miles to the southwest of the Strait of Hormuz.

If you had a combined problem of the Strait of Hormuz being largely closed and the Bab al-Mandab not being safe to passage, what's the impact on the world?

What's the impact on international shipping?

ARSENIO DOMINGUEZ: First of all, it's going to be a higher cost, and that's a reality.

So we will see an escalation not only of the cost, on the prices, but a detrimental effect on the seafarers, which will make it even more difficult for us to attract seafarers to the sector to be able to operate the ships.

So this is going to have a rolling effect, where the negative impact we will be feeling more and more, and for a long time.

NICK SCHIFRIN: Last night, Iran released an American detainee, Dena Karari, what President Trump called a gesture of goodwill.

But there is otherwise signs of bad will, and most U.S.

and regional officials predict the war will escalate.

For the "PBS News Hour," I'm Nick Schifrin.