U.S.-flagged ships have been advised to stay “as far as possible” from Iranian waters when navigating the Strait of Hormuz as tensions between Washington and Tehran remain elevated.

In a notice issued Monday, the U.S. Maritime Administration said ship captains should decline permission for Iranian forces to board U.S. vessels.

Boarding attempts, including moves to force commercial vessels into Iranian waters through small boats and helicopters, have occured as recently as Feb. 3, the agency under the Department of Transportation said.

Should Iranian forces board a U.S.-flagged commercial vessel, crews were advised not to “forcibly resist the boarding party,” the notice said. It added that refraining from forcible resistance does not imply consent or agreement to that boarding.

The advisory recommended that ships transiting eastbound in the Strait of Hormuz stay close to the Omani side of the waterway.