A Sudanese court has sentenced a Christian woman to death by stoning for alleged adultery, and religious freedom campaigners say the case helps explain why Christian minorities in many Muslim societies oppose the imposition of Islamic law.

New York-based Human Rights Watch has written to Sudan's president to appeal for the "cruel and inhuman" sentence to be lifted against 18-year-old Abok Alfa Akok, who was sentenced to death last December by a criminal court in south-western Sudan.

Apart from its established opposition to the death penalty, HRW also charged that Akok's trial was unfair. She had no legal representation; the trial was conducted in Arabic - not her tongue - without a translation of proceedings; and the man she's accused of having an affair with has not been tried for lack of evidence, it said. The case is now on appeal.

"The fact that only the woman has been tried for the crime of adultery particularly suggests discriminatory application of this harsh law," said Jemera Rone, Sudan researcher for HRW, in a statement.

"Imposing the death penalty in Arabic on this young woman who does not understand Arabic well constitutes a denial of her most fundamental human rights," Rone added.

In their letter to President Omar Hassan El Bashir, HRW Africa director Peter Takirambudde and women's rights division director LaShawn Jefferson wrote that stoning to death is a "painful and brutal" punishment.

They also noted that the International Covenant on Civil and Political Rights, ratified by Sudan in 1986, specifically prohibits the carrying out of capital punishment on a pregnant woman.

Stoning to death for adultery is one of a range of controversial physical punishments meted out under Islamic (shari'a) law. Numerous instances of stoning have been reported from Iran, and in recent years, several individuals have been sentenced to stoning in Nigerian states where shari'a has been imposed.

An exiled Iranian opposition group, the National Council of Resistance (NCRI), claims that since the 1979 Islamic revolution, "hundreds of women of various ages have been and continued to be stoned to death throughout Iran."

According to the NCRI, which has released secretly-filmed footage of such executions, women are buried up to their necks before being stoned to death.

"The stones should not be so large that a person dies after being hit with two of them, nor so small as to be defined as pebbles, but must cause severe injury," it says. "This makes it clear that the purpose of stoning is to inflict grievous pain on the victim, in a process leading to his or her slow death."

NCRI president Massoud Rajavi says stoning is not an Islamic punishment and the Quran makes no mention of it. It is, however, mentioned in the Hadith - the oral traditions attributed to Mohammed - and so is considered part of shari'a law, although some Islamic scholars dispute this also.

The Barnabas Fund, a British-based charity concerned with the interests of Christians under Islam, says what may be most disturbing of all about the Sudanese case is the fact that the condemned woman is a Christian, and therefore not supposed to be under Islamic law.

"The government of Sudan has in the past sometimes stated that Islamic shari'a law is only to be applied against Muslims," said Barnabas Fund director Dr. Patrick Sookhdeo.

"Yet Abok is a Christian from the Dinka tribe and should therefore not have been subject to Islamic law."

The case served as a warning that Islamic figures in Sudan and elsewhere in the Muslim world who call for the imposition of shari'a "fully intend that it should be imposed on Christians and other non-Muslims as well as Muslims, whatever they may say in public," he said.

"It is because of tragic cases like Abok's that Christians in Sudan, Nigeria and other parts of the Islamic world where the application of shari'a law is spreading have so consistently opposed it."

More than two million people are believed to have died during an 18-year civil war between the Islamic regime in Khartoum and the mostly animist and Christian south.

The U.S. Congress has defined the conflict as a "genocidal war" and human rights groups have recorded the widespread enslavement of southerners.