(Adds details on plant recovery)

ALGIERS, Aug 4 (Reuters) - Algeria's Tiguentourine gas plant has resumed full production for the first time since a militant attack in 2013, after its third train came back online, a senior source at state energy company Sonatrach said on Thursday.

The plant, operated with BP and Statoil with a full capacity of 9 billion cubic metres a year, bought its third train back to work on July 27, recovering from the Islamist militant assault and siege that killed 40 oil workers, the source said.

The increase at Tiguentourine, which supplied more than 10 percent of Algeria's total natural gas output before the attack, is a boost for the OPEC nation just as Algeria and the European Union are in talks about improving energy cooperation.

Algeria hopes dozens of projects, mostly in its southwestern Sahara, will generate new output and help keep its flow of gas to Europe stable longer term. But it has struggled to attract the investment needed to discover and develop new fields and maintain old ones.

(Reporting by Lamine Chikhi; writing by Patrick Markey; editing by Susan Thomas) ((pat.markey@thomsonreuters.com; +213-661-692993; Reuters Messaging: pat.markey.thomsonreuters.com@reuters.net))