A Vietnamese environmental activist was beaten in prison for refusing to confess his crimes, his wife told AFP Friday in the one-party communist state notorious for jailing critics.

Nguyen Ngoc Anh was sentenced to six years in jail in June for "undermining" the government on Facebook in posts about the environment and politics liked tens of thousands of times.

Vietnam has cracked down on online dissent since passing a controversial cybersecurity law last year, jailing dozens for content deemed critical of the government.

Anh is among dozens of activists behind bars in the country where political prisoners are kept in separate quarters to avoid influencing others.

His wife told AFP he was badly beaten by his cellmate this week.

"He could barely walk to see me... he could not use right hand to pick up food," Nguyen Thi Chau said after visiting him in jail in southern Ben Tre province on Friday.

Her husband said his arms, head and neck were injured and that he had been taken to the infirmary but was refused medicine.

She believes he was beaten because "the jailors wanted him to admit his crime but he did not".

The jail could not be reached and Vietnam's Ministry of Foreign of Affairs did not immediately return AFP's request for comment.

Though severe beatings are believed to be rare inside Vietnam's prisons, jailers may turn a blind eye or encourage cellmates to abuse activists, according to accounts from released prisoners.

Anh was a shrimp farmer in Vietnam's Mekong Delta and was active on Facebook, posting about politics and the environment -- including a massive fish kill in central Vietnam in 2016 blamed on a toxic dump by a Taiwanese steel firm.

Amnesty International called for his release.

"This brutal attack has added to his ordeal. It's a damning indictment of Vietnam's prison system," Amnesty's senior crisis adviser Joanne Mariner told AFP.

The organisation says there are at least 128 political prisoners behind bars in Vietnam, with about 10 percent jailed for Facebook posts even though the controversial cyber law has yet to come into effect.

The law will require internet providers like Facebook and Google to remove content and hand over user data if requested by the government.

Last month activist Nguyen Quoc Duc Vuong was jailed for posts and livestreams allegedly promoting Hong Kong protests and land rights in Vietnam, according to Human Rights Watch.