HARARE - Zimbabwe police have found foul play was not involved in a house fire that killed an opposition leader's son, state-backed media reported on Monday.

A 12-year-old boy who is the son of Shepherd Maisiri, a regional official of the Movement for Democratic Change (MDC), died last week when the hut he was in was burned in the rural town of Headlands.

MDC supporters said the blaze was caused by a firebomb thrown by activists from President Robert Mugabe's Zimbabwe African National Union-Patriotic Front (ZANU-PF).

"Forensic experts did their analysis combined with police investigations and no foul play is suspected," Charity Charamba was quoted as saying inthe Herald newspaper.

She said the fire and explosion could have been caused by bags of fertiliser and a paraffin lamp inside the hut.

"There is high probability that ammonium nitrate and tobacco chemicals exploded during the fire," she said.

The police did not explain how the fire was started.

Three other children who were in the hut at the time of the attack escaped with injuries.

The incident took place as the country gears for a referendum on a new constitution on March 16 and general elections slated for July, was condemned by the US embassy in Harare.

Past elections in Zimbabwe have been marred by violence including killings, assault and intimidation.

The violence that gripped the country's 2008 vote left at least 200 dead, most of them MDC supporters.

There have been sporadic cases of violence in recent weeks with MDC supporters being targets.