Uganda has failed to investigate the deaths of at least 40 people killed in two days of rioting three years ago, Human Rights Watch said in a statement Tuesday.

Dozens were killed when security forces in Kampala fired live rounds at unarmed protestors angry at authorities for blocking the cultural leader of the country's largest ethnic group from visiting part of his kingdom.

Despite several pledges from the government to investigate the killings, a parliamentary probe has made little headway and no security officers have been held accountable, the New York-based rights group said.

"The long government inaction on the killings of people in September 2009 is an insult to victims," said Maria Burnett, senior Africa researcher at Human Rights Watch.

"Resorting to lethal force without clear justification in the face of protests is unacceptable, yet it is becoming the norm in Uganda," Burnett said.

The government officially puts the death toll from the riots at 27 but hospital records and independent investigations show that over 40 people died, Human Rights Watch said.

Last year at least nine people were killed when security forces clamped down harshly on opposition protests against rising food and fuel prices.