Nigeria has warned civilians that Boko Haram is using deadly cluster bombs but campaigners say the Islamists may have got the banned weapons from the military in the first place.
Defence spokesman Rabe Abubakar urged people to be on their guard for "cluster bombs, sometimes scalled scatter bombs", as engineers had recently found caches in northeastern Adamawa state.
"The military high command has discovered that the Boko Haram terrorists in the areas have used such lethal instruments over time to push their callous terrorist cause," he said last Thursday.
"These bombs are used against large areas containing many targets such as columns of vehicles, marketplaces, places of worship or large troop concentrations," he added in a statement.
The insurgents, allied to ISIS group in Syria and Iraq, have in recent months increasingly used suicide and bomb attacks in northeast Nigeria, Chad, Cameroon and Niger.
Abuja has described the attacks, which have caused mass civilian casualties, as desperate acts following apparent army gains in the northeast when soldiers overran their training camps, and hundreds of fighters allegedly surrendered.
Cluster bombs are a category of ordnance dropped from planes or fired from artillery via a shell, missile or rocket and spread hundreds of tiny sub-munitions or "bomblets" over a wide area.
Many of the devices fail to explode on impact and effectively become de facto landmines hidden for years, making them difficult to clear and posing a significant risk to civilians.
More than 100 countries signed the UN Convention on Cluster Munitions that came into force on August 1, 2010 banning the use, production, transfer and stockpiling of cluster munitions.
Nigeria has signed but not ratified the convention and the Cluster Munition Coalition (CMC) campaign group said Abuja had still to declare its stockpiles and detail how it would get rid of them.
Photographs posted on the Nigerian Defence Headquarters Twitter feed suggested the bombs were sub-munitions for the French-made BLG-66 Belouga, normally launched from aircraft, the CMC said.
Nigerian pilots are said to have used the Belouga on bombing raids against rebel positions in Sierra Leone in October 1997, while the junta in Freetown at the time said civilians were also targeted.
Nigeria, which was then leading troops from the West African regional force ECOMOG seeking to restore the ousted Sierra Leone president Ahmad Tejan Kabbah, denied the claim.
CMC director Megan Burke told AFP in an email exchange it was unclear how Boko Haram was using the bombs, as the group does not operate aircraft.
But she added: "As to where BH would have gotten them, it seems possible that they could have come from Nigerian military stocks...
"Certainly we will be monitoring developments on this as this is a weapon that should not be used by anyone (government or non-state actor) under any circumstances."
Nigeria has previously indicated it still has old stocks of British-made BL-755 cluster bombs, which are launched from aircraft.
Ordnance expert Bob Seddon, a former head of bomb disposal for the British Army in Iraq and Afghanistan, also mentioned reports the Nigerian Air Force still has BL-755 munitions.
But he suggested the militants could also have acquired cluster bombs from the former Libyan stockpile and adapted them for use in IEDs, car and suicide bombs.
Smuggling routes from north Africa through the Sahel have been pinpointed as a likely source for Boko Haram weaponry, as well as looted armouries at Nigerian bases in the northeast.
A propaganda video published on social networks on Saturday showed Boko Haram fighters with a cache of what appeared to be military issue rifles, sub-machine guns and rocket-propelled grenades.
"We know that Islamic State in Libya has been making extensive use of munitions looted from (Khadafi)-era ammunition depots to make car bombs," Seddon wrote in an email.
"High explosive aircraft bombs are perfect for use in improvised IED main charges...
"It is possible that BH may have got their hands on some aircraft Libyan cluster munitions, the most common type being the Russian RBK-250.
"It is also possible that BH may have got their hands on some artillery or rocket delivered sub-munitions.
"The Chinese 122mm Type 84 rocket has been encountered in Libya and it delivers some very hazardous sub-munitions."
AFP emailed and texted Colonel Abubakar to ask whether Boko Haram's cluster bombs were home-made or commercially manufactured, where they may have got them and when they had been used.
He was also asked whether Nigeria still had a stockpile of the munitions. But there was no immediate response.