At first, the Islamic extremists in Nigeria's dusty north-east rode on the backs of motorcycles, firing on government officials and other perceived enemies with worn Kalashnikovs hidden beneath their flowing robes. Now, they come prepared for war.

When Islamic fighters drove into a town in north-eastern Nigeria last week, they used anti-aircraft guns, mounted on the backs of trucks, to destroy nearly every landmark of the nation's federal government. Fighters also rode in on at least one bus, the military said, while in other assaults insurgents have fired rocket-propelled grenades.

The militarisation of Islamic radicals in the north comes after witnesses saw Nigerian fighters mingle with the extremists who took over northern Mali in the weeks following a coup there. It also comes after fighters seized massive deposits of Nigerian military equipment and have gained access to arms smuggled out of the lawlessness of Libya.

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Those new arms, and the willingness of extremists to use them, highlight the increasing instability in Nigeria's north and the ever-growing dangers facing the nation's weak central government. With dozens dead in other incidents in the past few days, the violence in this country is mounting, but has yet to make headlines in the West. This is, in ways that Mali – with the intervention of French troops – was not, Africa's great silent bloodletting.

"Each year, they grow in prominence and sophistication," said David Zounmenou, an analyst at the Institute for Security Studies in Pretoria, South Africa. "That's what's making the fighting that much more difficult for the Nigerian security forces."

The sophistication of the fighters, who are most likely from the extremist network Boko Haram, could be seen in their assault on Bama, a town some 40 miles south-east of Maiduguri, the capital of Borno state. The military said some 200 fighters in buses and pickup trucks laid siege to the town. In their arsenal were truck-mounted anti-aircraft guns, weapons seen during the civil war in Libya and the recent fighting in northern Mali.

That attack in Bama killed at least 42 people, as well as 13 others that authorities described as Boko Haram fighters. The insurgents' heavy weapons helped them overrun the barracks of the Nigerian army's 202nd Battalion, as well as a police station, a police barracks, a magistrate's court, local government offices and a federal prison. The extremist fighters freed 105 prisoners during their assault, a Nigerian military spokesman, Lt-Col Sagir Musa, said.

The use of such weapons marks a transformation of Nigeria's Islamist insurgency, which grew out of a riot in 2009 led by Boko Haram members in Maiduguri. That ended in a military and police crackdown that killed 700 people. The group's leader died in police custody in an apparent execution-style murder, fuelling dissent that broke into the open in 2010 with the targeted killings of government officials, security agents and religious leaders who spoke out against Boko Haram.

Since then, Islamists have engaged in hit-and-run shootings and suicide bombings, attacks that have killed at least 1,618 people, according to an Associated Press count. That number doesn't include the killing in April this year of at least 187 people in the fishing village of Baga during fighting between extremists and security forces, as witnesses and human rights activists said Nigeria's military killed civilians and burned thousands of homes and businesses.

Casualties are expected to rise as the extremists now have access to sophisticated weaponry. While in the past decade Nigeria's military has fought against heavily armed militants and criminal gangs operating in the creeks of its oil-rich southern delta, analysts and security officials say those groups never had access to anti-aircraft weapons. Nor did those groups launch attacks on military barracks, or level towns.

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Where the weaponry has come from also remains unclear. A propaganda video released in March by Boko Haram, featuring its leader, Abubakar Shekau, showed fighters gathered around weapons they said they had stolen from an attack on an army barracks. Those weapons included what appeared to be heavy machine guns, rocket-propelled grenades and possibly anti-aircraft weapons, as well as ammunition and brand-new bulletproof vests.

Another source of tactics and weapons may come from northern Mali, where Nigerian extremists had joined the fighting.

"Boko Haram will also likely recruit militants who fought and obtained new skills from warfare in Mali," wrote the analyst Jacob Zenn in a recent publication by the Combating Terrorism Center at the US Army's West Point. "The Boko Haram attack on an army barracks in Monguno... in which the militants mounted weapons on four-wheel drive vehicles, and the discovery of improvised fighting vehicles in a raid on a Boko Haram hideout in Maiduguri... suggest that Boko Haram has already learned new methods of fighting from the Islamist militants in Mali."

Abubakar Shekau is a shadowy figure, now seen mainly in propaganda footage released by the group. Part theorist and part hoodlum, the former theology student is known to be taciturn, but other details remain murky. Even his age – said sometimes to be 34, on other occasions to be nearer 43 – is a matter of conjecture. And even his very existence has been disputed, as Nigerian authorities believed that he had been killed in 2009, only for him to re-emerge in Boko Haram videos.

Meanwhile, arms are likely to continue to come out of Libya from heavily armed militias there, Mr Zounmenou said. He explained that arms can be transported quickly through the Sahara and into West Africa's Sahel to Nigeria, a major shipment stop for illegal weapons.

While Nigeria's President Goodluck Jonathan has spoken before about the need to control arms shipments throughout West Africa, the trade continues largely unhindered. And as more of those weapons end up in the hands of Islamists in Nigeria's north, more violence can be expected.

"They are now really going to war," Mr Zounmenou said. It is not surprising that on Thursday, the Nigerian President abandoned a much-heralded diplomatic visit to southern Africa to return home to speak to security services about the increased body counts from violence across his nation.