They said a huge blast erupted at around midday (1100 GMT) in the crowded market in Gombi in Adamawa state, one of the worst-hit in the seven-year Boko Haram insurgency in the northeast.
"I heard a huge explosion coming from the grain section, which is at the edge of the market," trader Mustapha Jalo told AFP.
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"Many of us rushed to the scene and we found carnage. People were scattered everywhere. I can say over 10 people died in the explosion. I can't give precise number of the injured but there are many," he said.
Gombi, which lies about 120 kilometres (75 miles) from state capital Yola, has been hit several times by Boko Haram attacks, with the most recent taking place in March 2015.
Adamu Ahmad, a security guard at the market, gave a similar account, saying the bomber was believed to be a boy aged around 12.
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"There was a suicide explosion at the grain section of the market around midday. The explosion killed at least 11 people and injured several others," he said.
"The attack was believed to have been carried out by a boy of around 12. Today is market day in Gombi and the market attracts thousands of people from the district," he told AFP.
Following the blast, the market closed down and all the traders returned home, he said.
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Boko Haram, which is looking to establish a hard-line Islamic state in mainly Muslim northern Nigeria, has killed at least 17,000 people and made more than 2.6 million others homeless since its campaign began in 2009.
The group, which has pledged allegiance to the Islamic State (IS) group, has also carried out deadly attacks in neighbouring Chad, Cameroon and Niger.
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