By Liz Lagniton | 2026-04-27

Maritime safety concerns are rising after US forces intercepted three Iranian-flagged oil tankers -- the Deep Sea, the Sevin and the Dorena -- in congested Asian waters.

The vessels were operating near India, Malaysia and Sri Lanka in recent days when they were intercepted and redirected under US naval operations targeting Iranian oil shipments, according to Reuters, citing sources and ship tracking data on April 23.

The interceptions have raised concerns that poorly tracked tankers moving through busy Indo-Pacific shipping lanes can pose navigation risks, particularly where vessel identity and movements are difficult to verify in real time.

The Dorena, a fully loaded supertanker carrying about 2 million barrels of crude, was last tracked off southern India before coming under US monitoring and escort in the Indian Ocean.

The Deep Sea, a partially loaded supertanker, was last seen near Malaysia before dropping from public tracking systems for an extended period.

The Sevin, carrying about 65% of its roughly 1 million-barrel capacity, also operated with intermittent or disabled tracking before interception in the same broader region.

US Central Command (CENTCOM) confirmed aspects of the operation in a post on X on April 23, saying the Dorena "has been under the escort of a US Navy destroyer in the Indian Ocean after previously attempting to violate the blockade."

In a separate post, CENTCOM said the blockade on ships entering or leaving Iranian ports remains in effect and that US forces had redirected 34 vessels as of April 24 since enforcement began on April 13.

Dark fleet risks

US officials categorize these vessels as a "dark fleet" used for smuggling operations that bypass tracking requirements to remain undetected.

The risks posed by these vessels extend beyond mere invisibility. Experts warn that because these ships use opaque ownership and fraudulent registrations, there is no clear line of accountability in the event of a collision or spill.

An analysis by Elisabeth Braw at the Atlantic Council said the "large shadow fleet has come to pose a significant problem for coastal states and legally operating vessels," citing risks including collisions, hazardous spills and broader disruption to maritime order.

Tracking gaps

According to shipping data cited by Reuters, the Automatic Identification System (AIS) transponders on both the Deep Sea and Sevin had not been transmitting consistently for more than a month before interception. Some Iran-linked tankers have also been identified operating without verifiable registry or under shifting identities, compounding concerns over vessel accountability in international waters.

The practice of "going dark" obscures a tanker's course and speed in real time, leaving little margin for error in the congested corridors of the Indo-Pacific.

In busy lanes linking the Malacca Strait to India and Sri Lanka, this lack of visibility significantly heightens the risk of collision when nearby vessels cannot verify a ship's operational status.

US operations have extended beyond chokepoints such as the Strait of Hormuz into the Indian Ocean and Southeast Asian approaches, where major global shipping routes converge. A maritime security source told Reuters that US forces are operating in open waters to reduce risks in heavily contested chokepoints, including threats from naval mines.

CENTCOM has rejected suggestions that vessels are broadly evading enforcement, saying Iranian tankers Hero II and Hedy remain anchored in Chabahar, Iran, while the Dorena is under active escort.

"The US military has global reach," CENTCOM said, adding that enforcement operations continue across multiple maritime theaters.

Iran has previously condemned similar interdictions as unlawful seizures, accusing the United States of escalating maritime tensions and disrupting global energy flows.

Some vessels linked to Iranian exports are designated by the US Treasury's Office of Foreign Assets Control (OFAC), which has accused shipping networks of using complex ownership structures, falsified documentation and ship-to-ship transfers to evade sanctions.

Earlier US maritime operations this year targeted tankers suspected of carrying Iranian and Venezuelan crude as part of a broader effort to dismantle "shadow fleet" logistics. Key actions include the boarding of the Tifani in late April, the Bertha in late February, and both the Aquila II and Veronica III earlier that same month.