After days of razing villages and pitiless massacre,Boko Haram finished the week with its most chilling atrocity.

As people bustled through the Saturday market in the Nigerian city of Maiduguri, a device borne by a ten year-old girl exploded near the entrance.

A witness said the girl probably had no idea that a bomb had been strapped to her body.

The explosion just before lunch killed 20, including the girl, and injured 18, according to the police.

Boko Haram did not immediately claim the attack, but the Islamic insurgents have increasingly used young girls as human bombs as they carve an African “Caliphate” from the plains of northern Nigeria.

Today, Boko Haram controls about 20,000 square miles of territory - an area the size of Belgium.

Within this domain, the black flag of jihad flies over scores of towns and villages scattered across the neighbouring states of Borno and Yobe.

The latest conquest was the fishing town of Baga on the shores of Lake Chad, which fell to the Islamists last Wednesday.

“For five kilometres (three miles), I kept stepping on dead bodies until I reached Malam Karanti village, which was also deserted and burnt,” one surviving fisherman, Yanaye Grema, said.

^{The leader of the Islamist extremist group Boko Haram delivering a speech}

Boko Haram’s fighters have now achieved mastery over 11 local government areas with a total population exceeding 1.7 million people, according to the official 2006 census.

Once, the movement’s fighters would launch hit-and-run attacks on defenceless villages. Now, Boko Haram’s realm stretches from the Mandara Mountains on the eastern border with Cameroon to Lake Chad in the north and the Yedseram river in the west.

The Nigerian army, crippled by corruption and incompetence, has shown itself unable to resist the jihadist advance.

Last September, Abubakar Shekau, the self-styled “Emir” of Boko Haram, proclaimed his ambition to conquer a “Caliphate” and follow the example of the Islamic State of Iraq and the Levant (Isil).

^{The substantial control Boko Haram has is evident in this map}

Diplomats believe this was a logical escalation of Boko Haram’s campaign.

“There is a copy-cat element at work here,” said Andrew Pocock, the British High Commissioner to Nigeria. “If Isil can declare a Caliphate, then so can we. Boko Haram want to be seen by their peers as grown-up jihadis. They want to show 'we can control territory, we can control a Caliphate’.”

There is also a clear practical rationale for Boko Haram to capture territory. “Success - and they have had success - creates a different kind of requirement,” added Mr Pocock. “You need a place where you can base yourself and keep equipment and supplies and, indeed, captives. It means that you’ve got to hold territory.”

Shekau has established Boko Haram’s unofficial headquarters in the town of Gwoza in Borno state. This stronghold has been chosen with great care.

Gwoza is shielded from attack by the volcanic peaks of the Mandara Mountains spanning the nearby frontier with Cameroon. Most important of all, the surrounding area is the homeland of Shekau’s own ethnic group, the Kanuri.

From this base among his brethren, Shekau sends his fighters to strike across a vast area. The border with Cameroon means nothing to Shekau, since it slices directly through the area inhabited by the Kanuri. His men have frequently attacked villages in the neighbouring country, killing 68 of Cameroon’s soldiers in the last month alone.