Mrs Hill says her husband was willing to give-in to the demands

Margaret Hill, the daughter of an expatriate worker, was grabbed from a car on her way to school in the oil city of Port Harcourt.

Her mother, Oluchi, told the BBC that the kidnappers had called her demanding a meeting in a town in the Niger Delta.

She said they then allowed her to speak to her daughter who was crying.

Margaret was snatched by gunmen at 0730 (0630 GMT) on Thursday after they smashed a window of her car as it stood in traffic.

They say I can bring my husband to swap with the baby

Oluchi Hill

Oil rich city's woes Oil worker recalls kidnap

Her father Mike Hill, who has lived in the country for 10 years, runs a bar in Port Harcourt.

The region's main militant group - the Movement for the Emancipation of the Niger Delta (Mend) - has offered to help find the girl, according to the AP news agency.

"We will join in the hunt for the monsters who carried out this abduction and mete out adequate punishment for this crime - We abhor all forms of violence against women and children," the group said in an e-mail sent to AP.

Mrs Hill, a Nigerian national, said the kidnappers told her to meet them in a town in Bayelsa State in the Niger Delta region, but that neither she nor the police had been able to locate it.

"They say I can bring my husband to swap with the baby," she said. "He wanted to go down for his baby but the police commander told him not to."

The kidnappers then threatened to kill Margaret if Mr Hill did not come within three hours, she said.

After the deadline had expired, Mrs Hill said Margaret was being fed just "bread and water".

"The people who are holding her just called again and they were threatening to kill the baby," Oluchi Hill was quoted as saying by Reuters news agency.

"They accused me of trying to play games with them."

The UK's Foreign Office has called for the "immediate safe release" of the girl.

'Big business'

The BBC's Alex Last in Lagos says Mr Hill is ill and had been due to fly to the UK for medical treatment.

He says that no hostages had ever been killed by Nigeria's oil militants and that most situations are resolved after a ransom is paid.

MEND Formed early 2006 Close links to militant Mujahid Dokubo-Asari's Niger Delta Volunteer Force Split into two rival groups late 2006 Demand 100% control of Nigeria's oil wealth Demand release of impeached Bayelsa governor on trial for money laundering Operate from creeks of Niger Delta Communicate with media by e-mail

Nigeria's shadowy militants

Mend has said it had nothing to do with that attack.

Our correspondent says there are a plethora of armed gangs operating in the Niger Delta and kidnapping for ransom has become big business.

More than 100 foreign oil workers have been taken hostage in the region this year.

Mend called off its ceasefire on Tuesday, saying it has been kept on the sidelines of government-led talks about the future of the Niger Delta.

Although the Delta accounts for more than 90% of Nigeria's income, the region remains highly impoverished, a situation the militants say they want to change with their campaign.