ALGIERS, Jan. 16 (Xinhua) -- Algeria on Tuesday marked the fifth anniversary of a terrorist attack that targeted one of the country's biggest gas plants and killed 40 people.

The natural gas plant of Tiguentourine region, in the southernmost province of Ilizi, near the border with Libya, was attacked by a terrorist group affiliated to al-Qaeda on January 16, 2013.

The terrorists took control of the plant and held hundreds of workers from various nationalities as hostages. The terrorist attack left 40 hostages dead.

The plant is run in partnership between Algerian energy giant Sonatrach, British Petroleum (BP) and Statoil, and produced between 22 and 23 million cubic meters of natural gas per day.

Algeria's economy depends on oil industry, as investments in the energy sector hit 90 billion U.S. dollars in the period 2015-2019.

Although Algerian authorities have taken all the necessary measures to reinforce the security around and inside Tiguentourine plant, and in other sensitive facilities, the terrorist threat is still present, according to local observers.

Algeria faces ongoing terrorist threats as it is located in a region plagued by unprecedented security and political instability.

Tens of thousands of the country's troops are deployed on eastern and southern border to thwart potential intrusion of militants and arms, amid instability reigning in Mali and civil war hitting Libya.