Nigeria's militant Islamists have "completely surrounded" Maiduguri, the main city in north-eastern Borno state, traditional elders have warned.

The military needed to "fortify" the city, which had a population of more than two million, to prevent an assault "from all directions", they said.

The Boko Haram militants had "annexed" areas that were about 50km (30 miles) from Maiduguri, they said.

Boko Haram declared a caliphate in areas it controls last month.

The government has not yet commented on the statement issued by the Borno Elders Forum (BEF).

'Annihilation'

It represents influential people in the state, including former government ministers and civil servants.

Boko Haram has also captured territory in neighbouring Adamawa state, forcing people to flee into hills, where they are eating leaves, residents told the BBC.

"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," BEF said.

"The insurgents have rendered impassable almost all the roads leading to Maiduguri," it added.

Air strikes

BEF said the military needed to "urgently fortify" the city, where Boko Haram was founded in 2002.

"The insurgents have surrounded Maiduguri and are nursing the ambition of attacking the city from all directions," BEF said.

The violence has forced tens of thousands of people from their homes

Thousands of extra soldiers have failed to quell the five-year insurgency

The BBC's Bashir Sa'ad Abdullahi in the capital, Abuja, says tens of thousands of people are taking refuge in Maiduguri after fleeing Boko Haram's advance.

It is unclear what is happening in territory under their control in Borno, as the mobile telephone network in many places is down, he says.

In Adamawa, the military has launched an air and ground assault to recapture the town of Michika, seized by Boko Haram on Sunday.