At least 14 people have been killed in an explosion suspected to have been carried out by militant Islamists in north-eastern Nigeria, a regional senator has told the BBC.

Muhammed Ali Ndume said the bomb exploded near a military checkpoint in the town of Biu in Borno state.

In a separate incident, an opposition rally in southern Nigeria was hit by explosions and gunfire.

There are growing fears about stability in Nigeria ahead of elections.

The Nigerian Human Rights Commission says at least 58 people have been killed in in pre-election violence.

'Firing back'

Presidential and parliamentary elections were due to be held on 14 February but were postponed to 28 March after the military requested more time to curb the insurgency in the north-east.

The six-year conflict has killed thousands of people and has displaced more than three million, Nigerian officials say.

President Goodluck Jonathan, who is seeking re-election, has been strongly criticised for his handling of the insurgency.

He is facing a strong challenge from opposition leader Muhammadu Buhari, a former military ruler.

Soldiers returned fire in Biu, killing 17 insurgents, a security source told Reuters news agency.

The town was also targeted five days ago when a woman blew herself up at a crowded market, killing at least six people.

The attack in southern Nigeria targeted a rally organised by the All Progressives Congress (APC) - Nigeria's main opposition party - in Okrika, a small port town south of Port Harcourt in the heart of Nigeria's oil-producing region.

APC members were waiting for the rally to begin, witnesses said, when there was several explosions and gunfire.

About 50 people were reportedly wounded. It was not immediately clear who was responsible for the attack.

Boko Haram is not known to operate in the region.