The US said in an advisory on Monday that American-flagged ships should stay "as far as possible" from Iranian territory when passing through the Strait of Hormuz.
The advisory comes after Iranian naval forces reportedly harassed a US-flagged vessel in the area last week.
Iranian forces have historically used small boats and helicopters during boarding operations and have attempted to force commercial vessels into the country's territorial waters, the Department of Transportation said.
The Strait of Hormuz is a major route for shipping in the Middle East, particularly for tankers carrying oil to global markets. Iran has threatened to close it during times of geopolitical tension.
The Department of Transportation advises vessels to assert their rights under international law and decline Iranian forces permission to board, but to avoid resisting should they be boarded. To avoid such issues, it warned vessels to steer clear of Iranian waters "without compromising navigational safety".
“When transiting eastbound in the Strait of Hormuz, it is recommended that vessels transit close to Oman’s territorial sea,” it said.
Tension is simmering between the US and Iran. The US sent an "armada" to the Middle East in recent weeks in response to a government crackdown on protests in Iran. Concerns are high that Washington might launch another attack on Tehran.
Last week, US forces shot down an Iranian drone that ventured too close to the USS Abraham Lincoln, one of the American vessels set to the region.
Amid the tension, Iran and the US have held talks on the future of Tehran's nuclear programme. US President Donald Trump, who ordered strikes on Iran's nuclear infrastructure in June last year, described the talks in Oman as "very good" and added that negotiations would continue this week.
As the US warns ships of threats in the Strait of Hormuz, American forces continue to seize tankers believed to be carrying Venezuelan oil in the Caribbean in breach of sanctions. On Monday, the US announced it seized a vessel in the Indian Ocean, after it departed Venezuela weeks ago.
