Minna - A Nigerian Islamic sharia court on Wednesday postponed for almost a month the appeal of a young couple who were sentenced to be stoned to death.

Officials at Niger State's sharia Appeal Court said that the chief judge, Grand Khadi Abdumumumi Enagi, had "travelled out of town" and that the case had been put off until April 21. No explanation was given for his absence.

Fatima Usman and her former sweetheart Ahmadu Ibrahim, both now thought to be 33, were convicted of adultery in 2001 after Usman's father complained to a sharia judge that Ibrahim was not paying support for their illegitimate child.

The pair had been forewarned of the postponement and did not come to court.

Human rights lawyers say that the case is shot through with procedural errors and does not match Islamic law's, or sharia's, own strict rules of evidence. Niger state officials expect the couple to be cleared when the appeal is heard.

Niger is one of 12 northern Nigerian states to have reintroduced sharia principles into its penal code since the west African country returned to civilian rule in 1999 under a democratic federal constitution.

The move has proved extremely controversial both within and outside Nigeria. Riots between northern Nigeria's Christian-minority and huge Muslim community cost thousands of lives, and deep tensions remain in some northern cities.

Meanwhile, the reintroduction of stoning in cases of sexually related crimes - such as rape, sodomy and adultery - has triggered deep concerns among human rights and feminist groups at home and abroad.

The cases of village housewives Safiya Husseini and Amina Lawal, both sentenced to death for bearing children out of wedlock, caused an international outcry. Both were cleared on appeal, and no Nigerian has yet been stoned.

On Monday this week the case against a 23-year-old man, Jibrin Babaji, who was convicted of sodomy after confessing to molesting three young boys, was thrown out on appeal, after a sharia judge ruled there should have been four witnesses.

Nigeria is Africa's most populous country with more than 126 million people, just over half of them Muslims. Around 40 million are thought to live in the 12 sharia states, a broad swathe of territory on the southern edge of the Sahara.

The federal government of President Olusegun Obasanjo, himself a southern Christian, initially said it opposed sharia laws, but made no attempt to prevent it coming into effect.

In some regions the federal police now conducts investigations, arrests and prosecutions under penal codes based on a strict interpretation of Islamic law.