A Jordanian farmer was charged with premeditated murder after his unmarried pregnant sister was stabbed to death in the Jordan Valley on Thursday over an alleged affair, police said.

The unnamed 24-year-old "stabbed her 10 times in different parts of her body with a dagger in front of a group of people before handing himself over to police and confessing to the crime," a police spokesman told AFP.

"He heard his sister was having an affair with a man in Ghour Safi village and killed her without knowing the truth, claiming he wanted to defend his honour.

"Tests proved that the 27-year-old victim was four weeks pregnant, and apparently somebody told her brother about that."

The police spokesman said villagers "did not prevent the crime, but later tried to rescue the woman. They failed."

The suspect, who faces the death penalty if convicted, killed his sister near the family's home which is close to a mosque, he added.

Between 15 and 20 women are murdered each year in Jordan in the name of "honour." Last year, around 17 such killings were recorded.

On Tuesday a court in Amman halved the jail term of a 29-year-old man who shot dead his raped sister "in the name of honour," a judicial official said.

The criminal court initially sentenced him to 15 years with hard labour, but immediately reduced it to seven and a half years.

Parliament has refused to reform the penal code to ensure harsher penalties.