Karel Prinsloo, Associated Press
Kenya residents ride on the back of a police truck Sunday as they flee ethnic violence in Naivasha. Police said at least 19 people, including 11 children, were burned to death in a house by a mob.

NAIROBI, Kenya — Ethnically driven violence intensified in Kenya on Sunday, and police officials said at least 19 people, including 11 children, were burned to death in a house by a mob.

The country seems to be becoming increasingly unhinged, with even the Kenyan military, deployed for the first time, unable to stop the wave of revenge killings.

More than 100 people have been killed in the past four days, many of them shot with arrows, burned or hacked with machetes.

It is some of the worst fighting since a disputed election in December ignited long-simmering tensions that have so far claimed at least 750 lives. The fighting appeared to be spreading Sunday across the Rift Valley region, a particularly picturesque part of Kenya known more for its game parks and fancy lodges.

The Kenyan government is now threatening to arrest top opposition leaders on suspicion of orchestrating the bloodshed, but opposition leaders are in turn accusing the government of backing criminal gangs.

Police in Naivasha said fighting erupted Sunday morning between gangs of Kikuyus and Luos, two of Kenya's biggest ethnic groups, who have clashed across the country since the election. Witnesses said that mobs threw flaming tires and mountains of rocks into the streets to block police officers from entering some neighborhoods. The mobs then went house to house, looking for certain people.

Grace Kakai, a police commander in Naivasha, said a large crowd of Kikuyus chased a group of Luos through a slum, trapped them in a house, blocked the doors and set the house afire. Police found 19 bodies huddled in one room, and Kakai said that some of the children's bodies were so badly burned that they could not be identified.

"All I can say is that they were school age," she said.

"The situation is very bad," Kakai said. "People are fighting each other and trying to drive them out of the area. We have to evacuate people."

Thousands of families are streaming out of Naivasha, Nakuru, Molo, Eldoret and other towns across the Rift Valley. The region is home to supporters of both Mwai Kibaki, Kenya's president, and Raila Odinga, the top opposition leader, and the site of historic land disputes between members of rival ethnic groups.

Kibaki is a Kikuyu and Odinga is a Luo, and the disputed election, in which Kibaki was declared the winner by a narrow margin despite widespread evidence of vote rigging, set off the ethnically driven violence.