Region warns Niger's leader on plan to keep power
* Economic sanctions plus AU measures threatened
* Niger ECOWAS membership could be suspended
* Opposition calling for resistance to referendum
NIAMEY, May 18 (Reuters) - Niger's neighbours could penalise it with economic sanctions if President Mamadou Tandja's plan to stand for a third term contravenes pledges to follow democratic principles, regional body ECOWAS said on Monday.
The government announced this month the 70-year-old leader, whose second and final term expires this year, would hold a plebiscite on constitutional changes to allow him to run again in polls due in November. The move sparked a street protest and opposition anger in the uranium-mining state.
"All constitutional changes are banned, as is non-democratic accession or maintenance of power," the wise-men council of the 15-country Economic Community of West African States said in a statement.
Any violation of democratic commitments could lead the Community to suspend Niger's membership, "impose an economic embargo on it and even seek other forms of sanctions by the African Union", Abdel Fatau Musah, ECOWAS's director of political affairs, told national television.
ECOWAS's wise-men council is composed of veteran political figures and has a mediation role in regional disputes.
It did not spell out what measures could be taken against the country, one of the world's poorest, but which has attracted investment by French nuclear group Areva CEPFi.PA and others.
Tandja, who came to power in 1999 elections and won a second term with 66 percent of the vote five years later, is trying to end a two-year rebellion by nomadic Tuaregs in the uranium-mining north of the country which has destabilised swathes of the Sahara where al Qaeda cells also operate.
Some 20,000 people protested against the planned referendum this month in a rally organised by the opposition Niger Social Democratic Party, which has urged political parties, trade unions and civil society groups to resist the move.
Areva's Imouaren project, which is due to become Africa's biggest uranium mine, will cost an initial 1.2 billion euros ($1.62 billion) and is scheduled to start production in 2012. (Reporting by Abdoulaye Massalaki; writing by Mark John)