Gunmen have seized 15 children who were on their way to their international school in the south-eastern Nigerian state of Abia, say police. "The abductors have contacted [the owner of the private school] and asked for 20 million naira [$130,000 or £81,500]," police spokesman Geoffrey Ogbonna told AFP news agency. He said he believed the children came from wealthy Nigerian families. The kidnap happened on the fringes of the oil-rich Niger Delta on Monday. In recent years, gunmen in the Niger Delta have been kidnapping prominent Nigerians, and their relatives, rather than foreign oil workers, whose security has been improved. There has been a sharp rise in hostage-taking in Abia, but this is believed to be the first time so many children have been seized in a single group. The BBC's Caroline Duffield in Lagos says many middle class Nigerians in Abia state travel with armed escorts because of the threat. It is unclear whether the children's bus had security guards. Most hostages are released unharmed after a ransom is paid. Search underway The children were on their way to the Abayi International School in the state's commercial capital, Aba, when a vehicle blocked the path of their school bus, Mr Ogbonna said. He said the children - who attend nursery and primary school - were with a driver and teacher but were taken away by a group of armed men, whose identity is unknown. "We are making efforts to locate where they are held so as to free them," Mr Ogbonna said. A federal police spokesman in the capital Abuja, Emmanuel Ojukwu, said officers and investigators were on their way to the state to look into Monday's attack, reported AP news agency. Kidnappings in Nigeria's south-east are carried out by criminal gangs seeking ransom, but also by armed groups demanding a fairer distribution of oil revenue in a country flowing with oil but where most people live on less than $1 a day. An amnesty that came into effect in the Delta last year has reduced unrest in the region - though three French oil workers were abducted in a raid on an offshore drilling ship last week. Nigeria's parliament is considering a bill which would impose the death penalty on convicted kidnappers in a bid to deter would-be hostage-takers. Credit: BBC