Mohammad Souri, chairman and chief executive of NITC, has stepped down after 26 years at the helm just as Iran's biggest tanker group faces the threat of sanctions in the United States.

Shipping and oil industry sources said there was speculation Souri's departure could be linked to moves by Iranian President Mahmoud Ahmadinejad to entrench his authority in key strategic sectors, notably transportation and energy.

The National Iranian Tanker Company was privatised in 2000 and is owned by three Iranian pension funds. It is in the sanctions firing line with the U.S. senate discussing provisions that could blacklist the group.

"This serves to inform you that as of Jan. 30, 2012, I took retirement from National Iranian Tanker Company's CEO," Souri, a prominent figure in the global shipping industry, said in a letter sent to Reuters on Tuesday.

"Although I shall no longer be directly involved with NITC management, I shall be assisting as an adviser and supporter."

Hamid Behbahani, formerly Iran's road and transportation minister, was named as NITC's new boss.

"My successor has demonstrated an outstanding level of competence in various management areas," Souri said.

NITC confirmed that Souri had retired, declining to comment further. Other NITC officials could not be reached for comment.

Behbahani, who was impeached by parliament for mismanagement in February 2011, is a close ally of Ahmadinejad.

"The interesting point is Souri's replacement ... It could be part of the power struggle in Iran. There is talk that he (Souri) was supported by the supreme leader (Ayatollah Ali Khamenei) and efforts to replace him some months ago failed," an oil analyst in Iran said.

"It is also possible that as hardline rivals blame Iran's isolation and sanctions on Ahmadinejad's policies, Souri wanted to retire when the company faces problems because of sanctions, to put all future responsibilities on the president's shoulders."

Reuters