WASHINGTON (AFP) - The United States said Monday it was "deeply concerned" by the detention of the eldest daughter of Malaysian opposition leader Anwar Ibrahim.

Nurul Izzah, 34, who is a member of parliament, said she was arrested for sedition for reading out in parliament parts of a speech given by her father criticising his recent jailing.

Washington has voiced its concerns over her detention to the Malaysian government, State Department spokeswoman Jen Psaki said.

"The Malaysian government's recent investigations and charges of sedition against critics raise serious concerns about freedom of expression, rule of law and the independence of the judicial system in Malaysia," Psaki told reporters.

"To further restrict freedom of expression will only lead to further erosion of important pillars of Malaysia's democratic system."

She urged Malaysian authorities to ensure that they were fairly applying the rule of law and to take steps "to promote confidence in Malaysia's democracy, judiciary and economy".

Nurul was the latest to be rounded up in a sedition crackdown by Malaysia's government that has seen dozens investigated, charged or convicted over the past year, including several top opposition politicians.