Bomb detection units are investigating a "potential threat" inside a container on board a docked cargo ship at a port in Charleston, South Carolina, the US Coast Guard said late on Wednesday.
The terminal where the ship, the Maersk Memphis, is located has been evacuated while authorities look into unverified claims made online.
"A 1 NM (nautical mile) safety zone has been established around the vessel while law enforcement authorities investigate the threat," the Coast Guard said on Twitter.
The actions were taken out of an abundance of caution after a "conspiracy theorist" made a claim about a threat aboard the Memphis on YouTube, Coast Guard Sector Charleston spokesman Lt. James Zorn told local WCIV television.
A man named George Webb claimed the Memphis was carrying a "dirty bomb" after it had been loaded in Pakistan during a recent trip, he said.
However, there was no credible information the threat was real, Zorn added.
The threat came at around 8pm (1200 GMT Thursday), the Coast Guard said.