ALGIERS, Jan 14 (Reuters) - Algeria's army has arrested a cell of twelve militants in the south where they were planning an attack with the support of "terrorists based outside its southern borders", the ministry of defense said on Wednesday.

The ministry statement did not say where the militants were located or what was the potential target. But Algeria shares a vast southern border with northern Mali, where Islamist militants are active.

The statement did not provide details on the nationalities of the militants, but said the cell had been dismantled during operations since the start of January.

OPEC member Algeria has most of its oil and gas facilities based in its vast desert, and has tightened security along its southern borders since its major gas plant Amenas was attacked in early 2013 by militants.

Some of those fighters crossed over the frontier from southern Libya to carry out the attack that killed 39 foreign contractors at the plant.

Algeria, a key U.S. ally in its fight against Islamist militancy in the Sahel, is also concerned over fighters crossing onto its soil from Mali, Libya, and to a lesser extent Tunisia, where militants are also holed up in mountains on the border. (Reporting By Lamine Chikhi; editing by Patrick Markey and Ralph Boulton)