Algiers - A corruption investigation into Algeria's state energy firm is a warning signal from some in the state apparatus who believe part of the ruling elite has captured too much power, a former prime minister said.

The chief executive of Sonatrach, which supplies about 20 percent of Europe's gas needs, is under investigation along with several of the firm's senior management, in the biggest corruption case in Algeria for years.

Ahmed Benbitour, who was prime minister under President Abdelaziz Bouteflika until 2000 and also served as finance minister, told Reuters in an interview on Monday the Sonatrach case was part of a behind-the-scenes political struggle.

The Sonatrach affair is a "warning to those who want to capture the state", Benbitour said.

A decision in 2008 by parliament to amend the constitution, allowing Bouteflika, 72, to run for a third term in office was "one of the indicators of this process," he said.