Disruptions in and around the Strait of Hormuz threaten to further slow global shipments after US-Iran talks failed and Washington imposed a blockade on Iranian ports, experts say.

The United States military's blockade of ships "entering or departing Iranian ports and coastal areas" in the strait took effect on Monday. The strait links the Persian Gulf, the Gulf of Oman and the Arabian Sea beyond.

No vessels have made it past US naval forces during the first 48 hours of the blockade, according to US Central Command. Even so, the White House said the war in Iran was "very close to over" in an interview that aired on Wednesday.

The commander of Iran's joint military command threatened on Wednesday to halt trade in the Gulf region if the United States does not lift its blockade on Iranian ports.

At least 15 US warships formed the blockade, according to the United Kingdom Maritime Trade Operations, part of the UK Royal Navy.

Arsenio Dominguez, secretary-general of the International Maritime Organization, said 20,000 seafarers and around 1,600 vessels were stuck in the Persian Gulf on Monday.

The disruption is a major concern for shipping firms, businesses and oil traders. Brent crude, the international standard, rose to $96.32 a barrel on Thursday, well above its roughly $70 price before the war.

"The Strait of Hormuz (carries) approximately 20 million barrels of oil, which is 20 percent of the world's daily oil supply and one-third of seaborne oil supply that passes through this narrow waterway that is basically controlled by Iran," said Mohammad Elahee, a professor of international business at Quinnipiac University in Connecticut.

Iran exported 1.84 million barrels per day of crude oil last month. In April, it has so far shipped 1.71 million barrels per day, compared with an average of 1.68 million barrels per day last year, according to maritime data firm Kpler.

Trump once floated the idea of the US Navy escorting ships through the strait.

"That would be very hard," said Robert Kaufmann, an affiliate faculty member of the Boston University Global Development Policy Center. "The US Navy could, in theory, escort ships through there, but that would be expensive and slow."

Freedom of navigation

Stephane Dujarric, spokesman for the United Nations secretary-general, urged all sides to ensure freedom of navigation through the strait.

"The secretary-general's position has been consistent: No one should do anything that harms the freedom of navigation in the Strait of Hormuz," he said. "That freedom of navigation is based on international law and years and years of custom."

The strait is also a key global transportation route for fertilizer, meaning delays and disruptions will likely affect farmers.

"The cost of fertilizer has increased," said Luis Ribera, a professor and extension economist in the Department of Agricultural Economics at Texas A&M University.

"Slower shipments through the Strait of Hormuz make fertilizer prices increase more, both because of the slower shipments (and) the uncertainty," he said.

The UN Food and Agriculture Organization warned the crisis could lead to higher global food prices.

William Wilson, a professor of agribusiness and applied economics at North Dakota State University, said, "Everything going on in Hormuz is very negative to agriculture — negative for the world economy and for the world food economy in particular."

AP contributed to this story.