Nigerian government forces were fighting Islamist insurgents 35 km (20 miles) outside Borno state capital Maiduguri on Friday to try to stop any attack on the northeastern city, local residents said.

Authorities were struggling to reassure frightened locals that the armed forces would defend them against the Boko Haram militants, who have overrun a string of towns and villages in the area in recent weeks.

Maiduguri residents said they heard gunfire and explosions coming from the direction of Konduga, southeast of the city, on Friday, and later saw army troop carriers heading there, Reuters reported.

“Some people came from Konduga … they told us the army are in control,” Musa Sumail, a human rights activist in Maiduguri, told Reuters by phone. Other residents said they were told the army had intercepted an attempted probe into Konduga by a group of Boko Haram fighters. No details of casualties were available.

Sumail said military helicopters were flying over the Borno state capital, which has filled up with tens of thousands of refugees fleeing Boko Haram forces advancing from the north, east and south of Maiduguri in the last few weeks. Thousands more have fled Maiduguri westwards towards Damaturu for safety.

Some local civic organisations have warned that Maiduguri, where Boko Haram has concentrated its attacks since it launched its anti-government insurgency in 2009, is surrounded by the militants and vulnerable to attack.

Nigeria’s defence headquarters, which avoids giving detailed accounts of military operations, criticised such reports as “alarmist” in a statement on its Twitter account @DefenceInfoNG.

“All Facets of Security Arrangements for the Defence of Maiduguri has been upgraded to handle any planned attack,” the military said, without giving any specifics.

President Goodluck Jonathan’s administration and the armed forces face mounting criticism that they are failing in the war to counter Boko Haram. The group’s leader Abubakar Shekau proclaimed a “Muslim territory” in the northeast after seizing Gwoza near the border with Cameroon, to the east, last month.

“We are convinced that the Federal Government of Nigeria has not shown sufficient political will to fight Boko Haram and rescue us from the clutches of the insurgents which may ultimately lead to the total annihilation of the inhabitants of Borno,” the Borno Elders Forum, which groups dignitaries and elders from the northeast state, said in a statement.

It urged the government to “fortify” Maiduguri.

Boko Haram’s Shekau is apparently trying to follow the example of Islamic State in Iraq and Syria, which has declared its own caliphate. This strategy, which has seen Boko Haram hoist its flag over local government buildings in several towns and villages, departs from its usual hit-and-run tactics.