US-flagged ships should send their travel plans to American and British naval authorities if they intend to sail in the Gulf, the US Maritime Administration has said.
The seizure of commercial ships near the Strait of Hormuz - a narrow channel between Iran and the UAE - has unsettled shipping lanes that link Middle Eastern oil producers to global markets.
"Heightened military activity and increased political tensions in this region continue to pose serious threats to commercial vessels," said the US Maritime Administration (MARAD).
"Associated with these threats is a potential for miscalculation or misidentification that could lead to aggressive actions," it added.
Ships were also told to alert the US Navy's fifth fleet and the UK Maritime Trade Operations in the event of any incident or suspicious activity.
It warned they could face interference to their global positioning systems (GPS).
At least two commercial vessels since May 2019 have reported interference with their GPS and "spoofed" communications from unknown entities falsely claiming to be US warships.
Ships were also advised to refuse Iranian forces permission to board as long as it would not endanger the ship and crew - but that they should not forcibly resist.
The US, which has increased its military forces in the region, has blamed Iran for blasts on several tankers near the Strait of Hormuz, a charge which Tehran has continually denied.
Britain said on Monday that it was joining the US in a maritime security mission in the Gulf to protect vessels after Iran seized a British-flagged tanker.
UK vessels have become a likely target after Royal Marines seized an Iranian oil tanker, Grace 1, near Gibraltar on 4 July.
They said they suspected the ship was taking oil to Syria in breach of EU sanctions, but Iran called it an act of "piracy".
The Iranian coastguard detained the Panama-flagged tanker MT Riah on 13 July, accusing it, as with the latest seizure, of smuggling fuel.
Less than a week later, Iran's Revolutionary Guard Corps seized the British-flagged Stena Impero in the Strait of Hormuz.
Britain's P&O said it had cancelled cruises around Dubai and the Gulf because of the tensions.
Around a fifth of the world's oil passes through the strait.
It became the focus for a standoff between Iran and the US after President Trump pulled out of a 2015 nuclear deal and reimposed tough sanctions on Iran, severely restricting its lucrative oil exports.
Iran claims the responsibility of securing these waters lies with Tehran and other countries in the region.
"The maritime coalition that US is trying to form will create more instability and insecurity," said Iran defence minister Amir Hatami, according to the country's semi-official Tasnim news agency.
The US is lobbying other nations to join its coalition with Britain, which has the largest naval presence in the area after America.