The violence was fueled Tuesday when an opposition lawmaker, Mugabe Were, was shot dead outside his home.
The main opposition group urged calm. "This is a new level of escalation in the already terrible violence that we are seeing," Salim Lone, spokesman for the opposition Orange Democratic Movement, told Bloomberg News.
Militias from opposing ethnic groups are battling in several towns, and Kenyan army helicopters fired what police said were rubber bullets at crowds to disperse them, The New York Times reported.
The opposition lawmaker who was shot was working with leaders of different ethnic groups and was organizing a peace march the night before he died, according to the Times.
Violence over disputed Dec. 27 presidential elections, in which President Mwai Kibaki was deemed the winner, has exacerbated ethnic tensions and left at least 750 people dead and displaced about 250,000.
Official results of the election showed Kibaki, from the Kikuyu ethnic group, narrowly beating ODM leader Raila Odinga, a Luo. The electoral chief later said he was pressured to release the results and didn't know who actually won.
Annan is bringing together Kibaki and Odinga and said he hoped the immediate political issues could be resolved within four weeks and the longer-term issues settled within a year, Reuters reported.
Attacks between rival ethnic communities are escalating in the Rift Valley, west of Nairobi, said Linda Ochiel, program director at the Kenya National Commission on Human Rights, according to Bloomberg.
The violence has caused an estimated $1 billion in damage to Kenya's economy.