Sierra Leone has removed nine vessels from its shipping register after an investigation found they belonged to IRISL, Iran's embattled shipping line, the head of the West African state's maritime authority said on Saturday.

The move comes as Western powers seek to ground Iran's global oil carrier fleet by urging countries to deny their flags to Iranian ships, part of a broader push to curb Tehran's nuclear program with economic sanctions.

"The vessels were deleted as a result of the fact that the companies that registered the said vessels were actually subsidiaries owned and controlled by IRISL," Acting Executive Director of the Sierra Leone Maritime Administration Wurroh Jalloh told Reuters in an email.

He said four vessels - Alva, Amin, Tour and Benita - were removed from the register late last week and that five othershad been deleted since the end of August. He did not give further details.

Iran has been trying to reflag its ships in an attempt to avoid Western sanctions aimed at choking off its oil exports, prompting the United States to pressure countries around theworld to refuse any such requests.

In mid-August, Tanzania announced it was de-registering 36 Iranian vessels and dropping the Dubai-based shipping agent it said had flagged them without the east African government's knowledge.

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

The United States has a longstanding ban on imports of Iranian oil and has imposed new economic sanctions that have curbed Iranian oil imports by most other major nations.

The EU banned Iranian oil imports on July 1 and barred EU firms from transporting Iranian crude or insuring such shipments