Mr Tandja's critics accuse him of carrying out a coup
The US says Niger's President Mamadou Tandja has undermined a decade of good government in his attempts to stay in power beyond the legal limit.
The White House says it is concerned at the actions of the president, who has dissolved the constitutional court and parliament in his bid for a third term.
Nigeria and France have also urged the country to resolve the dispute.
Mr Tandja seized emergency powers last week, saying Niger's independence was under threat.
But opposition groups have accused him of carrying out a "coup d'etat". They called a general strike on Wednesday but this had a mixed response.
In a statement, the White House voiced concern over Mr Tandja's attempts to "rule by ordinance and decree and to dissolve the National Assembly and the Constitutional Court".
"These decisions undermine Niger's efforts over the last 10 years to advance good governance and the rule of law," the statement said.
"We are encouraged that the African Union has sent a delegation to Niger to attempt to find resolution to this political crisis.
"We will continue to consult with our partners in the region and monitor the situation in Niger closely."
French Foreign Minister Bernard Kouchner said the tensions caused by the president's actions were "dangerous".
He urged the 71-year-old leader to "respect the constitutional order".
France retains close links with many of its former colonies, including Niger.
A French company is currently planning to invest more than 1bn euros in Niger's uranium mines.
Meanwhile Niger's southern neighbour, Nigeria, has called for a peaceful resolution to the dispute.
The president, who has ruled the country for 10 years, is due to step down in December.
But he has scheduled a referendum for 4 August on changing the constitution to let him seek a third term in office.
The Constitutional Court ruled three times that the referendum was illegal without the support of parliament.
His supporters say Mr Tandja has brought economic growth to one of the world's poorest nations and so deserves the right to seek re-election.