KUALA LUMPUR (AFP) - Malaysia's opposition on Monday denounced what it called a government bid to increase the prison term of opposition leader Anwar Ibrahim in a controversial sodomy case.

Anwar, 66, was sentenced to five years' jail last month after a court overturned a 2012 acquittal on sodomy charges that he says are a false smear by Malaysia's authoritarian regime aimed at disqualifying him from politics.

Anwar is free pending an appeal to the Federal Court, Malaysia's highest.

But his lawyer Karpal Singh told AFP he received new court documents indicating prosecutors will push to increase the sentence by an unspecified level.

"As Anwar is too old for whipping, they must feel five years is too light," said Mr Karpal.

Malaysia's regime has long been accused of using the courts to harass the opposition, and the March 7 judgement sparked outrage in Malaysia.

The US State Department said it "raised a number of concerns regarding the rule of law and the independence of the court." The charges allege Anwar sodomised a male former aide. The appeals court said the lower court that acquitted Anwar had misjudged evidence.

In a statement on Monday, N. Surendran, a vice president of Anwar's People's Justice Party, called the case "a frontal assault" on democracy, denouncing the bid for a increased sentence as "selective, vicious and politically motivated".

Attorney-General Gani Patail abruptly hung up without commenting when reached by phone.

The ethnic Malay-controlled government that had dominated multi-cultural Malaysia since independence in 1957 has come under unprecedented pressure from a rising, multi-racial opposition led by Anwar.

The opposition shocked the government by winning the popular vote in elections last year for the first time. It failed to win a parliamentary majority due to what critics call decades of gerrymandering by the ruling regime.

Sodomy is punishable by up to 20 years imprisonment in Muslim-majority Malaysia.

Anwar faces jail, loss of his parliament seat, and disqualification from elections for another five years after his release, placing his career in serious jeopardy.

Anwar was being groomed to take over the government in the 1990s but was toppled in a rift with his then-boss, premier Mahathir Mohamad, and thrown in jail for six years on corruption and sodomy charges he calls bogus.