A top U.S. military commander said on Thursday Washington would work "aggressively" to find a solution to enable free passage for shipping in the crucial Strait of Hormuz.
Just hours later, Iranian state TV reported that Tehran had seized a foreign tanker smuggling fuel in the Gulf.
The Revolutionary Guards said the impounded vessel - which it has not yet named - was towed after it sent a distress call.
The U.S. blamed a series of recent attacks on fuel tankers on Iran.
Iran denies involvement, but has threatened to respond to new U.S. sanctions, imposed after President Donald Trump quit a landmark 2015 nuclear treaty which restricted Tehran's uranium enrichment.
This is the U.S.S.
Boxer, on patrol in the Arabian Sea, part of an increased American military presence and is asking allies to help protect the strategic waters off Iran and Yemen.
(SOUNDBITE) (English) COMMANDING OFFICER OF 11TH MARINE EXPEDITIONARY UNIT, COLONEL FRIDRIK FRIDRIKSON, SAYING: "Our mission is the same, it does not change, which is to promote security in the region, assure allies and certainly ensure the free flow of commerce throughout the straits and anywhere else." The amphibious assault ship carries not only Harrier fighter jets but an array of ground troops.
Three hovercraft, each loaded with three Light Armoured Vehicles, stand ready to launch, with trucks, Humvees and other gear lined up behind.
The U.S. Fifth Fleet, based in Bahrain, says Gulf Arab states have stepped up patrols as well.