Tanzania has asked Iran to ensure its oil tankers do not identify themselves as Tanzanian nearly a year after the east African country says it de-registered all Iranian vessels flying its national flag.

The maritime authority in Zanzibar, a semi-autonomous region of Tanzania, said it was concerned Iranian tankers appeared to still be using Tanzania's call sign as a prefix to their MMSI, a signal that identifies a ship and its location.

Zanzibar launched an investigation in July last year into accusations it had re-flagged 36 Iranian tankers. It concluded that a Dubai-based shipping agent had registered the vessels to fly the Tanzanian flag without its knowledge and insists it then de-registered the vessels.

But on Wednesday, Abdi Omar Maalim, director general of the Zanzibar Maritime Authority, said Zanzibar had written to Iran's Embassy after it received reports Iran's vessels were still showing on tracking systems as Tanzanian-flagged.

He added Zanzibar did not have its own evidence of this.

"The Zanzibar Ministry of Infrastructure and Communications has written a letter to the Iranian embassy in Tanzania to ask the Iranian government to tell its ships to stop using Tanzania's call sign," Maalim told Reuters.

Iran is subject to international sanctions, imposed against its nuclear program, and countries which are seen to help Iran flout these sanctions risk being punished.

Reflagging ships masks their ownership, which could make it easier for Iran to obtain insurance and financing for the cargoes, as well as to find buyers for the shipments without attracting attention from the United States and European Union.

It was not immediately possible to reach the Iranian Embassy Tanzania's commercial capital, Dar es Salaam.

Maalim said that a copy of the letter had been provided to the U.S. Embassy in Tanzania to demonstrate its commitment to dealing with the issue. But he said it was a problem Zanzibar could not tackle alone.

"We have asked international port authorities to help us identify Iranian ships that are still using the Tanzanian prefix," he said.

Zanzibar has its own maritime registry but vessels registered by it fly the national Tanzanian flag.