The US has urged the Burmese government to release the opposition leader, Aung Sang Suu Kyi, from house arrest "immediately and unconditionally".
Aung San Suu Kyi is under house arrest for the third time
A spokesman for President Bush said if the generals in Rangoon did not let her go, they would be backtracking on commitments to move towards democracy.
He added they could not indefinitely suppress the legitimate aspirations of the Burmese people.
It emerged earlier this week Aung Sang Suu Kyi's detention had been extended.
A spokesman for Aung San Suu Kyi's National League for Democracy said on Monday she had been told her house arrest would continue at least until next September.
"If true, this represents a return to a pattern of unacceptable backtracking on commitments the regime itself has made to move toward democracy and national
reconciliation," White House spokesman Scott McClellan said.
AUNG SAN SUU KYI
1990: National League for Democracy (NLD) wins general election while Suu Kyi under house arrest; military does not recognise the result
1991: Wins Nobel Peace Prize
1995: Released from house arrest, but movements restricted
2000-02: Second period of house arrest
May 2003: Detained after clash between NLD and government forces
Sep 2003 Allowed home after operation, but under effective house arrest
"The generals in Rangoon must come to understand that they cannot indefinitely suppress the legitimate aspirations of the Burmese people and resist the worldwide march to freedom and democracy," he added.
Aung San Suu Kyi has been held since May 2003 after clashes between her supporters and pro-government forces in northern Burma.
The reported extension comes after Burma's ruling junta pledged to release 9,000 prisoners and appointed a new Prime Minister, Soe Win.
Her party won a landslide victory in 1990 but has never been allowed to govern by the military, which has run the nation since 1962.