The US has warned of a potential threat to its merchant ships from Iran as tensions increased over sanctions.

US maritime administration MARAD said in an advisory that since early May the possibility of Iranian strikes had increased.

"Iran or its proxies could respond by targeting commercial vessels, including oil tankers, or US military vessels in the Red Sea, Bab-el-Mandeb Strait, or the Persian Gulf,” it added.

“Reporting indicates heightened Iranian readiness to conduct offensive operations against US forces and interests.”

US-Iranian tensions have risen since US president Donald Trump withdrew a year ago from a 2015 nuclear deal, Reuters reported.

MARAD said US-flagged ships were encouraged to contact the US Fifth Fleet at least two days before sailing through the Strait of Hormuz.

The semi-official Iranian ISNA news agency quoted hardliner Ayatollah Tabatabai-Nejad as saying: “Their billion(-dollar) fleet can be destroyed with one missile.

“If they attempt any move, they will ... (face) dozens of missiles because at that time (government) officials won’t be in charge to act cautiously, but instead things will be in the hands of our beloved leader [Ayatollah Ali Khamenei],” he said.