KATMANDU, Nepal: Authorities tightened security in a tense south Nepal district where recent rioting and clashes have killed at least 22 people, officials said Friday.

Residents of Nepal's southern plains have been demanding more rights, saying the government has sidelined their region in favor of the more populated, mountainous north.

Kapilbastu district's chief administrator, Narendra Dahal, said security has been stepped up there due to the potential for more trouble.

The latest violence erupted Sunday when gunmen killed a local politician. Unrest quickly spread throughout the area, 300 kilometers (180 miles) southwest of the capital, Katmandu.

A total of 22 people died this week in several disturbances around Kapilbastu, Dahal said.

Fighting has killed at least 90 people in the southern area this year as ethnic groups, some armed, have held protests and strikes to demand greater rights for residents of the region.

They are demanding a greater say in policy-making, and increased representation in both legislature and government positions.

Police who are investigating the violence could not say exactly who was battling whom in the past week's fighting, Dahal said.

The United Nations said Thursday that it was sending a human rights team to investigate the killings.

Nepal's government has set up a high-level commission, headed by appellate court judge Lokendra Mallik, to probe the violence and provide relief to victims of the southern strife.

Home Minister Krishna Prasad Sitaula was set to fly to the area on Friday in a bid to help control the situation, a statement from the ministry said.

A curfew was in effect in most parts of Kapilbastu district, though the highway that passes through it was open to traffic, Dahal said.