MAZAR-I-SHARIF, Afghanistan: An Afghan prisoner murdered his wife during a conjugal visit, allegedly because she had been unfaithful after he was jailed for killing her relatives, police said Wednesday.

Din Mohammad was sentenced to 20 years in prison two years ago for killing his mother-in-law, and his wife's brother and sister in the northern province of Samangan.

Mohammad's wife, whose name has not been disclosed, visited him in jail in the provincial capital Aybak on Monday and was found dead in a small private room used for inmates to see their wives, police said.

Mohammad confessed to her murder, Samangan police chief Ikram Nikzad told AFP.

"Mohammad was told by his mother that his wife had affairs," he said, citing a police investigation.

"When she came to visit him in jail he strangled her with her veil and killed her. They were meeting in a private prison room," he said.

Mohammad's mother has been detained for questioning, Nikzad added.

According to a UN report last month, Afghanistan has made progress in protecting women against violence, but many still suffer horrific abuse 11 years after a US-led invasion brought down the Taliban regime.

Afghan women still endure killings by relatives in the name of family honour, forced marriages and domestic abuses.

There are fears that legal protection and other gains made by women will be eroded after NATO and the United States withdraw most of their 100,000 troops by the end of 2014.