By Florence Tan SINGAPORE, April 16 (Reuters) - A second U.S.-sanctioned supertanker has entered the Gulf via the Strait of Hormuz, shipping data showed, despite a U.S. blockade on vessels visiting Iranian ports.

The U.S. Navy is imposing a sea blockade against Iran — the latest escalation in the war that has seen the flow of key energy supplies choked off at the Strait of Hormuz.

Iran's Fars News Agency says a supertanker sailed through international waters and the Strait of Hormuz with its tracking system switched on, "without any concealment."

The U.S. blockade of the Strait of Hormuz appeared to be working on Wednesday, as marine trackers reported no Iran-linked ships entering or exiting the strait since the blockade began on Monday.

The U.S. military's maritime blockade of Iran has "completely halted" sea-based trade with the Middle Eastern country, U.S. Central Command said.

The move sets off a high-stakes war of attrition that will test who has the higher threshold for pain—Tehran or global markets.

Marathon talks that failed to produce a breakthrough between the United States and Iran were not the final word in negotiations, officials in the US and the region said Monday, as the contours of the two sides’ disagreement — primarily over Tehran’s nuclear ambitions — came into sharper focus.