'We believe the detention today of prominent Malaysian opposition figure Anwar Ibrahim raises serious questions and concerns,' State Department spokesman Mr Sean McCormack said in a statement.
'We urge Malaysian authorities to resolve this matter in a manner that builds confidence in the impartial rule of law in Malaysia,' he said.
Mr Anwar underwent a long interrogation at the Malaysian police headquarters on Wednesday and was then taken to hospital for a medical examination and would be held overnight at an undisclosed location, his lawyers said.
Mr Anwar's wife Ms Wan Azizah Wan Ismail said she feared for his safety, after the events of a decade ago when he was sacked as deputy premier, beaten up by the police chief, and jailed for six years on sodomy and corruption charges.
Mr Anwar has dismissed the latest accusations as a conspiracy to prevent him from rising into power.
The charges threaten to derail his political comeback and his plans to oust the government with the help of defecting lawmakers after landmark March elections that handed the opposition unprecedented gains.
Two US rights groups also expressed concerns about the arrest and hoped the investigation into the sodomy claims was not used as a political tool to silence him.
The arrest 'is of great concern to us and the international community,' said Mr Lorne Craner, president of the International Republican Institute, and Mr Ken Wollack, president the National Democratic Institute.
They noted that the similar charge filed against him a decade ago was ultimately overturned by Malaysia's Supreme Court.
Mr Craner and Mr Wollack also questioned the timing of the current allegations amid Anwar's re-emergence on the national political scene since a ban on his involvement in politics expired in April 2008.
'We hope that the investigation into the allegation is not used as a political tool to silence an outspoken government critic,' they said.
Sodomy, even between consenting adults, is illegal in predominantly Muslim Malaysia and punishable by 20 years imprisonment. -- AFP
With imminent Oppositon takeover of the Federal government, the UMNO/BN govt is taking harsh and underhanded tactics to derail the Opposition takeover.