The Gulu General Hospital in Lapal, Niger State, said more than 20 staff were kidnapped, including patient relatives.
A security source said two people had been killed after the gunmen invaded the medical facility in large numbers.
Niger state governor, Sani Bello, did not specify how many had been killed.
Armed bandits operating for cash have kidnapped or killed hundreds across northwest Nigeria.
Officials there have also said that Islamist militant group Boko Haram had taken over multiple communities in the state, offering villagers money and incorporating them in their ranks to fight the government