An attack that saw two suicide bombers ram their car into a bus station in northern Nigeria has killed at least 22 people and wounded dozens more, police said Tuesday.

The attack on Monday in Kano, the largest city in Nigeria's mainly Muslim north, led to a huge explosion that hit five buses, police spokesman Magaji Majia told AFP.

President Michel Suleiman met with his Nigerian counterpart Goodluck Jonathan on Monday and the the two leaders discussed the purported execution by Islamist radicals of seven foreign hostages this month, including two Lebanese.

Suleiman said he still had hope "the kidnappers will set (the hostages) free immediately".

President Michel Suleiman headed on Tuesday to Africa on an 8-day tour at the head of a delegation, the state-run National News Agency reported.

The President will hold talks with senior African officials in Senegal, Ghana, Ivory Coast and Nigeria in addition to the Lebanese community in those countries.

Foreign Minister Adnan Mansour denied on Monday reports that the Lebanese hostages, kidnapped in Nigeria in February, have been killed, in light of the emergence of a video allegedly showing the victims.

He told LBCI television that none of the individuals shown in the video are the Lebanese hostages.

Britain, Italy and Greece on Sunday said that a claim by a Nigerian Islamist group that it had killed seven foreign hostages appeared to be true, while London denied it had sought to stage a rescue.

Restive northern Nigeria has seen scores of people killed from Islamist-linked violence, but the hostage executions marked the deadliest kidnapping targeting foreigners in the region in recent memory.

Nigerian Islamist group Ansaru on Saturday claimed to have killed seven foreign hostages – including two Lebanese men -- abducted from a construction site last month in the country's restive north, SITE Intelligence Group said.

There was however no confirmation from Nigerian authorities or the countries where the hostages were said to come from. An official from the Lebanese-Nigerian construction company, Setraco, told Agence France Presse he was aware of the report but could not confirm it.

Gunmen have shot dead a senior police officer who was visiting his home state of Enugu in southeast Nigeria, but the motive and identity of the suspects remained unclear, police said Monday.

Chinwike Asadu, the police commissioner of Kwara state in central Nigeria, was killed on Saturday night by gunmen in a bus who trailed his car and shot him as his driver was entering his compound, Enugu state police spokesman Ebere Amaraizu told Agence France Presse.

Nigeria's military said Sunday that it killed 20 Boko Haram Islamists while repelling an attack by the extremist group in the embattled northeastern state of Borno.

"Boko Haram terrorists attempted to attack a military barracks (in Borno) at about 5:00 am (0400 GMT)," spokesman Lieutenant Colonel Sagir Musa said in a statement.

Gunmen have stormed a residence in central Nigeria that housed Lebanese workers, killing two police guards in an attack a company source said Thursday was a failed kidnapping.

Police in Taraba state confirmed the raid in the village of Tella at the site owned by the Triacta engineering firm, but declined to provide details, including whether the three Lebanese engineers working there were the target.

Six foreigners kidnapped on February 17 by armed pirates from an oil service ship off Nigeria have been released unhurt without a ransom being paid, police said on Tuesday.

"All the six foreign hostages (were) released Monday evening unhurt. No ransom was paid before their release," said the police commissioner in Bayelsa state, Kingsley Omire.