* U.N. adds to international criticism of Tandja's plan

* Power extension bid undermining democracy

DAKAR, July 2 (Reuters) - The United Nations added its voice on Thursday to growing international criticism of actions taken by Niger's President Mamadou Tandja to extend his rule in the uranium mining West African country.

"The Secretary-General is deeply concerned about the ongoing political and constitutional crisis in Niger, which threatens to destabilize the country and undermine the progress made in recent years to consolidate democratic governance and the rule of law," a spokesman for Ban Ki-moon said in a statement.

On Wednesday, the White House said Tandja's plan to hold a referendum that would enable him to rule beyond his second term in office, which is due to expire later this year, was undermining the rule of law.

Tandja has said the population of Niger want him to remain in power to oversee major projects. French energy firm Areva <CEPFi.PA> is building what would be Africa's biggest uranium mine, while China's state-owned petrochemicals company signed a $5 billion oil deal last year.

Former colonial power France on Tuesday criticised Tandja's dissolution of the constitutional court, the body which had declared the planned referendum unlawful, while the European Union said on the same day that Tandja's moves could threaten aid to the impoverished desert country.

Niger's main opposition leader, Mahamadou Issoufou, was briefly detained on Tuesday after calling on the military to disobey orders, while opposition calls for a general strike in Niger were largely ignored on Wednesday. (Reporting by Daniel Magnowski; Editing by Louise Ireland)

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