At least 40 people have been killed in two separate attacks by members of the militant group Boko Haram in northeastern Nigeria since last week, a government official says.

On Monday, Baba Shehu said that around 70 gunmen attacked the town of Bama in Borno state late on Thursday.

"They shot ... 27 persons and injured 12. About 300 houses were burnt," Shehu said, adding, "Immediately people saw the gunmen, they (locals) started fleeing the town but the insurgents opened fire on them."

The government official further noted that in a separate attack on Saturday, 13 people travelling on a bus in the same area were ambushed by the militants and “murdered in cold blood."

"The attackers took the travelers unawares ... Some of the passengers escaped unhurt from the vehicle but were pursued by the insurgents and killed," he added.

Violence has intensified in northeastern Nigeria since President Goodluck Jonathan ordered his security forces in May to crush Boko Haram's four-year-old rebellion.

Nigerian authorities have cut off telephone lines in Borno state in an attempt to disrupt Boko Haram's attacks. Therefore, it often takes days for news of violence to reach state capital Maiduguri where reporters are based.

The Nigerian forces have been raiding militant camps and launching air and ground attacks on suspected hideouts of the militants over the past two months.

Boko Haram says its aim is to topple the Nigerian government, which it accuses of being pro-Western.

The group has claimed responsibility for a number of deadly gun and bomb attacks in various parts of Nigeria since 2009.

Over the past four years, violence in the north of Africa’s most populous country has claimed the lives of 3,600 people, including killings by the security forces.

In May 2013, the Nigerian government imposed a state of emergency in three states in the northeast, saying Boko Haram had become a security threat in those states.