ADDIS ABABA (Reuters) - Two Somali suicide bombers who had planned to kill football fans during Ethiopia's World Cup qualifying match against Nigeria on Sunday blew themselves up accidentally before carrying out the attack, the Ethiopian government said on Monday.

The explosion on Sunday happened in the upscale Bole district of the capital, about 5 km (3 miles) from Addis Ababa Stadium where thousands of fans were gathering for the match.

"They were Somali nationals and plotted to carry out a suicide attack disguised as fans on either the stadium or areas where large crowds gathered to watch the game," government spokesman Shimeles Kemal said.

Explosives, arms, hand grenades and football shirts were found at the scene of the blast, at a residential address an hour before kick-off, Shimeles said. Three suspects were detained. The Bole district has a small Somali refugee community.

"Heavy police presence must have caused their nervousness and state of agitation which led to the detonation that caused their own death," Shimeles said.

There was no immediate claim of responsibility.

Somalia's al Shabaab Islamist rebels have vowed to exact revenge on Ethiopia for sending troops to Somalia to fight the al Qaeda-linked militants, alongside African Union forces from Uganda, Burundi and Kenya.

Ethiopia says it has thwarted attacks over the past two years which it has blamed on domestic rebel groups as well as the Somali insurgents.

However, it has so far been spared the sorts of attacks carried out in nearby countries - such as the siege at the Nairobi mall last month and an attack on football fans in Uganda in 2010.

(Reporting by Aaron Maasho; Editing by Drazen Jorgic and Robin Pomeroy)

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