Security forces are searching for a family of seven French tourists kidnapped by suspected Islamist militants in Cameroon three days ago and taken into Nigeria, police say.
There has been a surge in clashes in recent days between suspected members of the armed rebel group Boko Haram and the military in Nigeria's northeastern town of Maiduguri, near the border with Cameroon.
"There is a massive manhunt ongoing," National Police Spokesman Frank Mba told Reuters on Friday. "Security operatives are working around the clock with search and surveillance to solve this."
The French hostages and kidnappers were near a small town called Dikwa at one point on Thursday, a Nigerian military source in Maiduguri told the Reuters news agency, asking not to be identified.
Dikwa is around 80km from Maiduguri and about the same distance to the border with Cameroon, where the three adults and four children were taken hostage on Tuesday.
Cameroon authorities said the victims, who were visiting a national park at the time, were taken over the border into Nigeria's restive northeast after being abducted.
French President Francois Hollande said on Thursday the hostages had probably been separated.
French police backed by special forces arrived in northern Cameroon on Wednesday to help locate the family, a local governor and French defence ministry official said.
The abduction was the first case of foreigners being seized in the mostly Muslim north of Cameroon, a former French colony, and highlighted the threat to French interests in West Africa since Paris deployed thousands of troops to Mali to oust al Qaeda-linked rebels who controlled the country's north.
On Sunday, seven foreigners were snatched from the compound of Lebanese construction company Setraco in northern Nigeria's Bauchi state, and another armed religious group, Ansaru, took responsibility.
Boko Haram frequently clashes with security forces in its stronghold Maiduguri but witnesses said there has been a surge in attacks in the last three days.
The military in Maiduguri declined to comment.
Many people were killed when suspected members of Boko Haram blew up a customs office, destroyed roadside stalls and fought gunfights with the military on Thursday, three witnesses and a military source said.
"After the explosion the Boko Haram started sporadic shooting with rapid propelled guns leading to the death of many people," a commander in the military Joint Task Force told Reuters, asking not to be named.
Two corpses lay outside a police station on Friday, believed to be those of fighters, witness Aminu Hakuri said.
The French foreign ministry has urged citizens in the far north of Cameroon to leave the area as quickly as possible and advised against travel to areas bordering Nigeria until further notice.
It could not say how many French citizens are believed to be in the north but 6,200 in total are registered as living in Cameroon.
France had already strongly advised its citizens against travel to northern Nigeria, which has seen violence linked to Boko Haram's fight against the government for several years.