Abuja, march 09 (Reuters) -- Nigeria must prosecute those behind a weekend massacre and address underlying issues of poverty and discrimination if it is to end a cycle of violence in the zone between its Muslim north and Christian south, rights groups and diplomats said.

The United Nations, United States, rights groups and opposition politicians all urged the authorities to ensure those responsible face justice after attacks on Sunday on three Christian villages in which hundreds are feared to have died.

Residents of Dogo Nahawa, Zot and Ratsat, about 15 km (9 miles) south of the central city of Jos, buried dozens of bodies including those of women and children in a mass grave on Monday following the attacks, which they blamed on Muslim herders.

The raids were in apparent retaliation for four days of violence around Jos, the capital of Plateau state, in January which killed several hundred people, many of them in an attack on the mostly Muslim settlement of Kuru Karama.

"Better security is clearly vital but it would be a mistake to paint this purely as sectarian or ethnic violence, and to treat it solely as a security issue," U.N. High Commissioner for Human Rights Navi Pillay said.

"What is most needed is a concerted effort to tackle the underlying causes of the repeated outbreaks of ethnic and religious violence which Nigeria has witnessed in recent years, namely discrimination, poverty and disputes over land."

The latest unrest at the heart of Africa's most populous nation comes at a turbulent time, with Acting President Goodluck Jonathan trying to assert his authority while ailing President Umaru Yar'Adua remains too sick to govern.

Jonathan deployed troops to quell January's unrest and pledged that those found to have "engineered, encouraged or fanned" the violence would be brought to justice.

But a dusk-to-dawn curfew was still in place when Sunday's attack took place. Some villagers were hacked to death with machetes as they tried to flee their homes after hearing gunfire. Others were burned alive.

U.S.-based Human Rights Watch said the military deployment had been limited to major roads and failed to protect small communities. It called for a credible investigation into what it said had been a massacre of at least 200 Christian villagers.

"This kind of terrible violence has left thousands dead in Plateau state in the past decade, but no one has been held accountable," said Corinne Dufka, senior West Africa researcher at Human Rights Watch. "It's time to draw a line in the sand."

U.N. Secretary-General Ban Ki-moon called on political and religious leaders to find a "permanent solution" to the crisis in Jos. The United States urged the government to ensure those responsible faced justice in a "transparent manner".

"CROCODILE TEARS"

Plateau state lies at the crossroads of Nigeria's Muslim north and Christian south and fierce competition for control of fertile farmlands between indigenous groups and settlers from the north have repeatedly triggered unrest over the past decade.

Its position on Nigeria's main ethnic and religious fault line means it is viewed as a microcosm of the wider country, a patchwork of more than 200 ethnic groups.

The instability underscores the fragility of Africa's top energy producer as it approaches the campaign period for 2011 elections with uncertainty over who is really in charge.

"The killings are more political than religious ... The government is the problem. It has the power of arrest and prosecution. It has the ability and resources to gather intelligence," the opposition Action Congress party said.

"Concrete action to stop the cycle of impunity, rather than crocodile tears, will end the violence," it said.

Police spokesman Mohammed Lerama said 93 people had been arrested after Sunday's violence.

But Plateau state has been here before. Large numbers of arrests have not translated into large numbers of prosecutions.

More than 300 people were arrested in January and about half of them were due to be sent to the capital Abuja for prosecution, but it is unclear how many actually faced justice.

Local officials said many of those responsible for January's violence were the same people arrested but not prosecuted after similar unrest in November 2008.

Many of Nigeria's prisons are overcrowded and the legal system overburdened with cases. It is not uncommon for communities to punish criminals themselves and blame their actions on the country's weak judicial system.