As of 1500 GMT on Tuesday, tracking platform Kpler had detected just four crossings of the strategic waterway that day by vessels carrying raw materials.

Five were detected on Monday -- a similar rate to traffic during the week before the US-Iran agreement to end the Middle East war, announced late Sunday.

"The Strait of Hormuz continues to operate below normal commercial levels, despite signals of diplomatic progress," Kpler said on X.

The Iranian government said on Tuesday that the US military's counter-blockade of Iranian ports, imposed on April 13, had been lifted ahead of the formal signing of the agreement.

US President Donald Trump said on Monday that ships were starting to come out of the strait. He said it would "completely" reopen as soon as the accord is signed on Friday.

Shipping groups have warned however that uncertainty remains about the steps towards reopening and how long it could for the strait to be cleared of mines.

"Key operational questions remain unresolved, including transit security, navigation fees and safe passage arrangements," Kpler said.

The vessels counted since the deal all transited with their transponders switched on but others may have passed without transmitting signals, making detection more difficult.

An Iranian tanker, the Diona, switched its transponder back on Tuesday for the first time in nearly two months.

Tanker owners' association Intertanko said in an advisory published on June 5 that the US Navy had been liaising with ships to help them safely pass by night, with their lights and transponders off, through the strait on its southern side near Oman's coast.

It said 15 ships a day had been using that route.

Before the conflict erupted on February 28, about 120 transits a day were recorded, according to maritime news outlet Lloyd's List.

In peacetime the strait saw around a fifth of global oil and liquefied natural gas exports pass through, along with other key commodities.