Suspected Boko Haram gunmen launched a series of attacks in Nigeria's restive northeast on Tuesday, opening fire on troops in one town before raiding a French-owned factory in another, witnesses said.

The violence in Gombe state came just after midday and began with a gunfight at a military checkpoint on the outskirts of Nafada town.

A convoy of suspected Boko Haram gunmen carrying grenade launchers attacked the security services, a bank and a political party office in Nigeria's restive northeast on Tuesday, witnesses said.

The attack in Gombe state came just after midday and began with a gunfight at a military checkpoint on the outskirts of Nafada town.

A large explosion on Monday rocked a major Shiite ceremony in northeast Nigeria's Yobe state, an area repeatedly attacked by Sunni Islamist group Boko Haram, multiple witnesses said.

The targeted worshipers were gathered outside a seminary in the Tsohuwar Kasuwa area of Potiskum city after visiting the palace of the local emir, said witness Ibrahim Maina and a police officer who requested anonymity.

Gunmen armed with dynamite freed at least 50 inmates in a raid on a prison in central Nigeria's Kogi state, the same facility attacked by Boko Haram Islamists in 2012, police said Monday.

"They blew up the thing with dynamite," national police spokesman Emmanuel Ojukwu told AFP of the attack which took place late Sunday. "Up to 50 inmates escaped," he said, but added that some have been re-arrested.

A triple bombing at a bus station in north Nigeria during Friday morning rush hour killed at least eight people and injured 41 others, as hopes of a ceasefire with Boko Haram appeared to suffer another blow.

It is not clear if the Islamist militants were behind the attack in Gombe city, the capital of Gombe state, but the extremists have a track record of targeting commuters.

A German national who was kidnapped by gunmen last week in southwest Nigeria has been released, his employer, the construction firm Julius Berger, said on Thursday.

"Julius Berger Nigeria Plc is pleased to officially announce the release of the company's member of staff, who was abducted in Ogun State on 24 October 2014," a statement from the firm said.

Boko Haram fighters on Wednesday clashed with Nigerian troops in the country's remote northeast, forcing thousands to flee, after two other attacks that further undermined claims of a ceasefire.

Residents said insurgents first overran the town of Uba in Borno state before moving to try to take Mubi, a commercial hub in neighboring Adamawa, using firepower including rocket-propelled grenades.

A pathologist on Tuesday ruled out claims of an explosion as the cause of a fatal building collapse at the church of a popular Nigerian preacher, saying none of the victims had blast injuries.

"Bodies were mutilated, severe crush, head injuries, fractured bones, fractured ribs," Lagos state chief medical examiner Professor John Obafunwa told a coroner's inquest in the city.

Gunmen on a speedboat in Nigeria's oil-producing south killed four policemen and kidnapped six Nigerian employees of the Agip energy firm, in the latest unrest to hit the restive region, the security forces said Tuesday.

The attack happened on Friday in the Nembe Creek area of Bayelsa state, which was one of the most violent regions of the delta at the height of an insurgency that formally ended in 2009.

Gunmen killed one German national and abducted another after opening fire on two vehicles in southwest Nigeria, police said Monday. 

The attack happened Friday morning near the town of Sagamu in Ogun state, with details of the shooting emerging as German Foreign Minister Frank-Walter Steinmeier visited the Nigerian capital Abuja.