At least four policemen were killed in an attack by unknown gunmen on a police station in north-eastern Nigeria late on Friday, the Borno state police command said.
"Commissioner of Police [...] expresses heartfelt condolences to the families of four gallant police officers who tragically lost their lives while defending the Police Quarters in Gajiram Town, Nganzai LGA," the statement said.
The police also advised the public to be vigilant and report any suspicious activity to the relevant authorities immediately.
Security problems have been common in Nigeria recently, with sporadic attacks and kidnappings by Islami extremists.
A day earlier, media reported that in another Nigerian state, Kwara, in the west of the country, assailants invaded the palace of the traditional ruler, Oba Aremu Olusegun Cole, where they murdered him and abducted his wife and another person.
Two more traditional rulers were shot dead by unidentified gunmen in neighboring Ekiti state on Monday, and several students and teachers were kidnapped, Nigerian police said.
To combat the security challenge, Governor Dauda Lawan of the northwestern state of Zamfara mobilized 4,200 civilian volunteers on Wednesday.
Boko Haram has been known for kidnappings and killings since 2002. In March 2015, Boko Haram swore allegiance to the terrorist group Daesh* and changed its name to the Islamic State West Africa Province (ISWAP). Boko Haram and ISWAP are now two separate groups.
* Terrorist organizations banned in Russia and many other states.