ALGIERS, Jan 30 (Reuters) - Algeria's state energy firm Sonatrach on Monday launched invitations for international engineering firms to bid on four major petrochemical plants worth a total of up to $6 billion, a source at state energy firm Sonatrach told Reuters.

The firms selected will carry out construction work on the four refineries located in Tiaret, Hassi Messaoud, and two in Skikda, including a fuel oil cracking plant, and another Nafta processing plant.

The plants in Tiaret and Hassi Messaoud will have 5 million tonnes capacity each, the source said.

OPEC member Algeria, hit hard by the crash in oil prices, plans to build more refineries to boost its revenues. It currently produces 30 million tonnes of oil refined products per year.

(Reporting by Lamine Chikhi; Editing by Ruth Pitchford) ((lamine.chikhi@thomsonreuters.com;))