Two bomb blasts have ripped through mosques in northeast Nigeria, killing at least 55 people and injuring more than 100.
The Friday attacks in Maiduguri, Yola and the Cameroonian town of Kerawa underline the ongoing threat from Boko Haram militancy in the region, authorities say.
Fears have been heightened particularly in Maiduguri, which has been hit six times this month, with a reported total of 76 people killed.
Questions are also being raised about how the militants are able to carry out such attacks on a regular basis, after similar attacks in the city last month claimed 117 lives.
The bombings additionally demonstrate the challenges facing the United States, which last week announced the deployment of up to 300 military personnel to northern Cameroon.
The contingent will conduct surveillance and intelligence operations against Boko Haram, including within Nigeria, at a time when attacks on civilians are on the rise.
The first of Friday's attacks, in Maiduguri, happened just after 5am in the Jidari area of the Borno state capital.
Umar Sani, a civilian vigilante assisting the military in the counter-insurgency, and local resident Musa Sheriff both told news agency AFP there were two blasts at the mosque.
"I was involved in the evacuation. We counted 28 dead bodies apart from the two bombers, who were identifiable by the mutilation of their bodies," Sani said.
"Over 20 other people were injured."
Nigeria's National Emergency Management Agency (NEMA) said only six people were killed and 17 others injured, while hospital sources put the death toll at 19.
Both Sani and Sheriff said two other people were arrested and handed over to the military for questioning after they were seen apparently celebrating following the blasts.
The two men were "standing from afar, hugging each other like a celebration, chanting "Allahu Akbar" (God is greatest)", said Sani.
"To them it was a mission accomplished," added Sheriff.
Boko Haram, which wants to create a hardline Islamic state in northeast Nigeria, has previously targeted mosques and religious leaders who do not share their extremist ideology.
Nigeria's President Muhammadu Buhari has given his military commanders until December to end the insurgency, which has left at least 17,000 people dead and more than 2.5 million homeless since 2009.
The explosion in Yola happened about 2pm at the Jambutu Juma'at mosque in the Jimeta area of the city, shortly after the imam had finished his inaugural sermon.
At least 27 people were killed in the blast at the newly-inaugurated mosque, NEMA said.
NEMA co-ordinator Sa'ad Bello said 116 people were being treated for injuries at two hospitals in the city.
Most were stable, with injuries ranging from fractures and burns to cuts, he added.
In Cameroon, regional and security sources said Boko Haram rebels had briefly overrun the town of Kerawa, in the far north, and an unspecified number of civilians were killed.
"They pulled out after the troops arrived. There hasn't been any more fighting," a source said, while another said the Islamist group had "fled" to neighbouring Nigeria.