Iran has in the past threatened to close down the Strait of Hormuz, a portion of which lies within its territorial waters, and has at times seized commercial ships and oil tankers moving through the area, alleging smuggling.

The US Department of Transportation’s Maritime Administration advised US-flagged commercial vessels to stay as far from Iran’s territorial waters as possible and to verbally decline Iranian forces’ permission to board if asked, according to the guidance.

“It is recommended that US-flagged commercial vessels transiting these waters remain as far as possible from Iran’s territorial sea without compromising navigational safety,” according to the guidance posted on its website.

It also said crews should not forcibly resist Iranian forces if they board.

“If Iranian forces board a US-flagged commercial vessel, the crew should not forcibly resist the boarding party,” it said.

Iran’s top diplomat said on Friday that nuclear talks with the US mediated by Oman were off to a good start and set to continue, in remarks that could help allay concern that failure to reach a deal might nudge the Middle East closer to war.

While both sides have indicated readiness to revive diplomacy over Tehran’s long-running nuclear dispute with the West, Washington has said it also wants the talks to cover Iran’s ballistic missiles, support for armed groups around the region, and human rights.

President Donald Trump ratcheted up the pressure on Iran on Friday with an executive order imposing a 25% tariff on imports from any country that “directly or indirectly” purchases goods from Iran, following through on a threat he made last month.