Imprisoned Malaysian opposition leader Anwar Ibrahim was prosecuted for political reasons and should be released, a UN body has concluded after reviewing a case widely seen as politically motivated.

Anwar began serving a five-year jail sentence in February after the country's top court ruled that he sodomised a former male aide in 2008.

Critics say his case was intended to eliminate any threats to the ruling coalition, whose popularity has eroded in the last two elections.

The UN Working Group on Arbitrary Detention found Anwar was denied a fair trial and his jailing was politically motivated.

It called for his immediate release, according to a letter from the group seen on Monday. Its full report is not yet available.

The government has rejected the findings and called for the legal process to be respected.

Anwar's daughter, Nurul Izzah Anwar, said she hoped the government will abide by the group's conclusion.

"I am deeply grateful that the United Nations has called for Anwar's release," said Nurul Izzah, a politician.

"Its strong stance in solidarity with my father sends a clear and unequivocal message to Prime Minister Najib Razak, and ensures that the sharp decline in human rights under his administration will not go unnoticed."

The government reiterated that Anwar's trial was "a criminal, not a political case" as it was brought against him by a private individual.

It also said the final verdict was reached by the courts after an exhaustive and comprehensive legal process over many years.

"It had nothing to do with the government," the prime minister's office said in a statement.

"Malaysia has an independent judiciary - with many rulings going against senior government figures - and the government does not have the power or authority to overrule the decisions of the courts."

It urged all parties to respect "the legal process, the judgment of Malaysia's courts and the rights of the victim to seek justice".

'Deep concern'

The working group is appointed by the UN Human Rights Council and its five current members are from Australia, Benin, Mexico, South Korea and Ukraine.

The independent body is mandated to consider and render opinions about alleged cases of arbitrary detention.

In the letter, the group also expressed concerns about Anwar's physical and psychological health amid allegations that he is being held in solitary confinement.

It said the Malaysian government did not respond to its queries on the case.

The US State Department on Monday reiterated its "deep concern" that Anwar's detention was apparently politically motivated.

Spokeswoman Elizabeth Trudeau said that Secretary of State John Kerry had raised the issue with Malaysia's Deputy Prime Minister Zahid Hamidi when he visited Washington in October.

Anwar led his alliance to unprecedented gains in 2008 elections and made further inroads in 2013 polls.

The ruling National Front coalition won with a slimmer majority and lost the popular vote to the opposition.

Source: AP