Five explosions at a bus park in northern Nigeria's main city of Kano killed at least 25 people on Monday, a Reuters witness said, in an area where Islamist sect Boko Haram is waging an insurgency against the government.
The coordinated bombing came as an audio tape emerged of a man saying he was the father of a family of seven French tourists kidnapped by Boko Haram militants.
On the tape he read out a threat by them to increase kidnappings and suicide bombings in Cameroon, if authorities there detain more of the group's followers.
Boko Haram, which wants to carve an Islamic state out of Nigeria, has killed hundreds in gun and bomb attacks since it intensified its insurgency two years ago.
The sect and other related Islamist groups have become a threat to Nigeria, Africa's top oil producer, and Western interests there, and are increasingly menacing its neighbours like Cameroon.
The blasts in Kano destroyed several buses in the Sabon Gari area, mostly inhabited by immigrants from Nigeria's largely Christian south, the Reuters witness said. Military and police cordoned off the area after the blasts.
There was no immediate claim of responsibility, but Boko Haram, which has frequently attacked the city, was a prime suspect. A spokesman for security forces in Kano state, Ikedehia Iwehia, said dead and wounded were being evacuated.
The sect often targets Christians.
"I ran for my dear life and managed to get out of the park after the second blast. Many people are lying dead. See, my clothes are covered in blood," said witness Ibrahim Bello, holding up a blood-soaked shirt.
The French family was kidnapped from north Cameroon last month but is believed to be being held in Nigeria. Boko Haram has a presence in Nigeria, Cameroon, Niger and Chad, where the four meet on the threshold of the Sahara.
Expanding attacks in Cameroon, a major oil, coffee and cocoa exporter, would further destabilise the region.
In the tape obtained by Reuters on Monday and whose authenticity was being checked by the French Foreign Ministry, the man believed to be Tanguy Moulin-Fournier appealed to Cameroon to free Boko Haram prisoners as a condition of his family's release.
"They don't want to enter in conflict with Cameroon. However, if you arrest their men again in Cameroon, they will multiply kidnapping and suicide bombing operations more in Cameroon than in Nigeria," he said.
"We have been detained for 25 days. The living conditions are harsh and hot in the desert. We are losing strength every day and are becoming sick. We cannot stay long like this," he said, adding that his youngest child was only four.
The tape is preceded by a message in the northern Hausa language by a man claiming to be Boko Haram leader Abubakar Shekau, saying unless Nigeria and Cameroon release prisoners from the sect, the French hostages will never be set free.
French Foreign Minister Laurent Fabious visited Nigeria and Cameroon over the weekend to discuss the hostage crisis. Eight French nationals are being held in northern Nigeria - the family plus another being held by Islamist group Ansaru.
The proliferation of kidnappings in parts of northern Nigeria and its neighbours have highlighted the growing risk posed by violent Islamist groups to Western interests.
Western governments fear ties with groups elsewhere in the region are drawing Nigerian Islamists towards a more explicitly anti-Western agenda, like that of al Qaeda's north African wing, especially since France launched an operation to flush them out of northern Mali in January.
Nigeria's President Goodluck Jonathan said on Monday that some of the seven other hostages believed to have been killed by Islamist group Ansaru this month might actually still be alive and the government has been working to rescue them.