Police say they have found the location where 19 people bound for Nigeria’s national service programme are being held, but fear an assault could trigger explosives planted around the site.
Commissioner Audu Bosso said surveillance drones pinpointed the hideout.
Officers have not revealed its location or named any suspects.
Bulldozers and armoured vehicles are being brought in as preparations continue for an operation to free the hostages.
The trainees were seized last Thursday near Umunoha, in Imo state, while travelling from Ibadan to induction centres in Abia and Akwa Ibom.
Their journey was for the compulsory year of public service undertaken by most Nigerian university and polytechnic graduates.
Omotosho Adewale, whose child was among those taken, said the abductors had sought 50 million naira, about $37,680 or £28,440.
He told the BBC he could not raise the money and had been unable to eat for three days. He appealed to officials to intervene.
Bosso said police intelligence suggested the compound was rigged with improvised explosive devices, making a direct entry dangerous.
Abductions for payment remain a serious problem across Nigeria, carried out by criminal gangs and armed factions.
Large-scale incidents are less frequent in the south than in some northern regions, and the latest case has prompted concern about the safety of young people travelling for national service.
The National Youth Service Corps says it avoids posting participants to places assessed as dangerous because of violence or kidnapping threats.
Most of south-eastern Nigeria has not previously been designated high risk.
Some earlier abductions in the region have been associated with separatist movements, including the Indigenous People of Biafra.
Officials have made no connection between any group and the current case.
By: Magdalene Agyeiwaa Sarpong