WASHINGTON (AFP) - The United States issued a rare stern rebuke to friend and ally Malaysia on Wednesday over what it said were attempts to muzzle the free press.

The south-east Asian nation is in political crisis, amid corruption allegations against Prime Minister Najib Razak, and has cracked down on media reporting.

The State Department criticised Malaysia's "recent actions to restrict access to domestic and international reporting on Malaysian current affairs."

A statement, signed by spokesman John Kirby, cited a decision last month to block access to news site The Malaysian Insider as cause for concern.

And he complained that Malaysia "has not acted transparently nor provided due process" in targeting media groups, journalists and social media users.

"Malaysian officials have also publicly described coming amendments to its Communications and Multimedia Act that would further restrict online space," he said.

While hailing Washington and Kuala Lumpur's close ties, he urged Malaysia "to ensure that all its laws, existing and future, fully respect freedom of expression."

Najib is under pressure over allegations that billions of dollars had been pilfered from state firms and that he has had hundreds of millions in his bank account.

This week, the Wall Street Journal, citing foreign investigators, reported that the prime minister may have received as much as US$1 billion (S$1.4 billion) from abroad.

Najib's office denounced the report, but he has previously acknowledged that he received a multi-million-dollar "personal donation" from Saudi Arabia.