A state of emergency declared in Nigeria in areas hit by attacks blamed on Islamist group Boko Haram has sparked concerns that the military will use the measure as legal cover to carry out further abuses.

A military task force in Borno state, Boko Haram's stronghold, has been accused a number of times in recent months of killing civilians and burning homes after bomb attacks, alleging residents collaborated with the militants.

Soldiers and police have also faced accusations in the past of gunning down civilians and carrying out summary executions in central Plateau state, which has been hit by years of clashes between Christian and Muslim ethnic groups.

Such actions have only worsened the situation and turned residents who might not otherwise support the extremists against the military, some analysts say.

The emergency declaration in parts of four states, including Borno and Plateau, gives security agencies extra powers to search and arrest.

"They've already been committing abuses," said Jibrin Ibrahim of the Centre for Democracy and Development think-tank in Abuja. "It will just legalise it, in a sense."

President Goodluck Jonathan declared the state of emergency on Saturday in response to scores of attacks attributed to Boko Haram, particularly bombings on Christmas that killed 49 people, most of them in a gruesome blast at a Catholic church as services were ending.

While Boko Haram has been carrying out increasingly deadly attacks for months, including an August suicide bombing of UN headquarters in Abuja that left 25 dead, the Christmas violence triggered intense fear and outrage.

It also led to warnings from Christian leaders that they would defend themselves if such attacks continued, raising more worries in a country roughly divided between a mainly Muslim north and predominantly Christian south.

Amid mounting frustration over authorities' failure to stop the violence, Jonathan announced the measures and declared that Boko Haram would be crushed.

The concern that has been expressed over the military and police is whether they will engage in killings, round-ups and invasions of people's homes.

"We see it as a blank cheque given to the military to perpetrate all sorts of abuses in the name of bringing an end to the violence," said Shehu Sani, a northern-based activist and head of the Civil Rights Congress of Nigeria.

"We are of the belief that as a democracy, Nigeria's government must fight the insurgency within the ambit of the law and respect for fundamental rights of citizens."

But there are also questions over what the emergency declaration will mean in practice apart from closing borders in hard-hit areas, which began on Monday.

Military spokesmen did not want to discuss details of the operations, but one said training had occurred to address past abuses.

"We have reviewed and we continue to review our modus operandi as far as this fight against terrorists is concerned," said army spokesman Major General Raphael Isa.

Nigeria's military, West Africa's largest, has a long history of abuses -- though it has at the same time gained respect as an important supplier of troops to international peacekeeping missions.

The country has also seen six military coups since independence in 1960, but the emergency decree applies to such a small portion of the vast country analysts said they did not see any such danger under the current situation.

Nigeria, Africa's most populous nation and largest oil producer, has been under civilian rule since 1999.

Some who voiced their support for the emergency declaration also hoped that it would be accompanied by dialogue with the militants.

Tanko Yakasai, a respected veteran northern politician, called the decree a "necessary evil" and said he believed the government would keep the military on a tight leash to keep the situation from spiralling further out of control.

"I believe the government will make sure that the military will handle the situation more professionally, with more care, realising the implication of misbehaving in this kind of situation," he said.

Others were not as sure and questioned why an emergency decree was even necessary, with military task forces already operating in a number of violence-hit areas.

Speaking of Maiduguri, the capital of Borno state, Ibrahim said that "the whole city has been militarised and that did not have an impact on what was going on."

"I don't see how the declaration of emergency on its own would have an impact," he said.