A prominent shipping organisation has condemned the United States and Iran’s tit-for-tat capture of commercial ships and is calling for the immediate release of their crews.

In an interview with Al Jazeera, John Stawpert, marine director of the International Chamber of Shipping, said seafarers must be allowed to go about their business “freely and without persecution”.

Stawpert, whose organisation is the top trade association for merchant shipowners and operators worldwide, called the capture of the vessels an affront to freedom of navigation as enshrined in international law.

“All these people are doing is transporting trade. And really, we can’t have a situation where ships are being seized, ultimately for political ends, to prove a political point,” said Stawpert, whose organisation represents about 80 percent of the world’s merchant fleet.

“These are innocent farers and they should be allowed to go about their jobs without fear of, essentially, imprisonment.”

Stawpert said Iran’s stated wish to charge tolls in the Strait of Hormuz had no basis in international law and would set a dangerous precedent.

“If you can do it in the Strait of Hormuz, why can’t you do it in the Strait of Gibraltar, say, or the Straits of Malacca?” he asked.

Stawpert also said the US President Donald Trump’s naval blockade of Iranian ports had heaped further uncertainty on shipping companies already reeling from Iran’s effective closure of the strait.

“We don’t know what conditions are in place. We don’t know what the targeting criteria of Iran are really,” Stawpert said. “And so we then have another state coming in, effectively doing the same thing through the blockade of the straits”.

The US and Iranian militaries have each announced the capture of two commercial vessels over the past week as Washington and Tehran continue to face off in the strait and in waters beyond the Gulf.

The US defence department on Thursday said it had captured the Iran-linked Majestic X as it was transporting sanctioned oil in the Indian Ocean, days after announcing the interception of another ship, Tifani.

Iran’s Islamic Revolutionary Guard Corps on Wednesday said it seized the Panamanian-flagged MSC Francesca and the Greek-owned Epaminondas for “operating without the necessary permits and tampering with navigation systems”.

The Philippines’ Department of Migrant Workers on Wednesday confirmed 15 Filipino seafarers were on the two vessels.

Officials said they had been assured by Iranian authorities that all the crew were “unharmed” and “safe.”

Montenegro’s maritime minister, Filip Radulovic, said in an interview with the state broadcaster earlier this week that four Montenegrin crew on the MSC Francesca were “fine”.

There have been no official updates on the condition of the crews on the vessels captured by US forces.

“It seems they’re not being maltreated,” Stawpert said. “But even so, that’s not really the point. The point is they shouldn’t be in custody in the first place”.

Stawpert also expressed concern for the well-being of an estimated 20,000 seafarers who have been left stranded in the Gulf due to the effective closure of the strait.

“Their welfare is also a priority for us,” he said. “The psychological burden, I think, will be beginning to tell on them after seven weeks now of what’s, to all intents and purposes, house arrest”.

Stawpert called on both the US and Iran to respect freedom of navigation.

“Let’s resume freedom of navigation and respect the right to innocent passage as soon as we possibly can,” he said.

The blockage of the strait, which usually carries about one-fifth of global oil and natural gas supplies, has driven up fuel prices worldwide and forced many governments to start emergency energy-saving measures.

Traffic in the waterway remains a fraction of pre-war levels, with reports saying just five ships transited the strait in the last 24 hours.

Before the US and Israel launched their war against Iran on February 28, the strait saw a daily average of 129 transits, according to the United Nations Trade and Development.