Maiduguri - Authorities imposed a 24-hour curfew in Maiduguri on Monday after suspected Islamist militants staged another attack there.
Baba Ahmed Jidda, a spokesperson for the government of Borno State, of which Maiduguri is the capital, announced the curfew in a statement, but provided no details about the attack and did not say whether there had been any casualties.
"The imposition of the curfew is necessitated by an attack in Maiduguri by people suspected to be Boko Haram members in the early hours of today," he said.
"Borno state government pleads with all citizens to remain calm and law abiding until the situation is put under control."
The Nigerian military has periodically imposed curfews on Maiduguri, birthplace of the Boko Haram insurgency, but it is rare for the state government to make such an announcement.
In May, President Goodluck Jonathan declared a state of emergency in Borno and two other states, ordering an all-out offensive against the Islamist group, which is fighting to establish an Islamic state in Nigeria, Africa's most populous country split roughly evenly between Christians and Muslims.
Initially the campaign appeared to temper the violence as soldiers wrested back control of towns, cities and stretches of semi-desert in the northeast from the militants.
But they have retreated to remoter areas, including the forested Gwoza hills near Cameroon, from where they have mounted deadly counter-attacks and have stepped up killings of civilians they accuse of collaborating with the authorities.
The military said last Friday it may have killed more than 50 Islamist insurgents in an air strike on one of their main bases in Gwoza.