On Saturday, the ministry had said the cholera toll was 14 in the town of Maiduguri, with most of the victims living in a camp for people displaced by Boko Haram violence.
"As of 4 September, 21 deaths have been reported," the Borno state health ministry said Tuesday, as the toll mounts.
"The total number of suspected cholera cases is now 375 in Maiduguri," it said, up from 186 at the weekend.
The government and NGOs are working to contain the outbreak which has spread to the town of Dikwa, 60 kilometres away, where five cases had been reported, it added.
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Maiduguri, the birthplace of Boko Haram has been repeatedly attacked by the jihadists.
The city's population has doubled since the start of the eight-year Boko Haram violence, rising to some two million inhabitants due to influxes of displaced people from across the state.
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