Four United States soldiers and five civilians on boats were detained and interrogated by Iranians for several hours on Sunday, according to US Central Command.
A dispute over the Shatt al-Arab sparked the 1980s Iran-Iraq war
They were sailing up the Shatt al-Arab waterway when they were taken by force, a spokesman for Central Command said on Monday.
The soldiers and three of the civilians, including an oil contractor, were escorted back to their boats and released on Monday, the spokesman added.
The nationalities of the civilians are unclear.
They were en route to the Mini al-Bakr platform to conduct a survey when the Iranians took the crew and blindfolded them, the spokesman said.
They were then taken to a building where they were interrogated on Sunday night.
On Monday the Iranians took the group back to their boats and released all of them except the two drivers, Central Command said.
A Chinook helicopter located the group near the waterway, and US Navy soldiers drove the boats to Kuwait.
Initial medical examinations indicate there were no injuries or signs of physical abuse.
The group may have moved into Iranian territorial waters by mistake, the spokesman said.
The Mini al-Bakr platform is close to Iran's international water.
A border dispute between Iraq and Iran over the Shatt al-Arab was one of reasons that sparked the eight-year war between the two countries in 1980.