The three-year-old daughter of a British expatriate worker has been kidnapped by gunmen in the Niger Delta, Nigerian police have said.
Militants say they have been left out of talks on the Delta's future
The child, Margaret Hill, was seized from a car on her way to school in the oil city of Port Harcourt.
The kidnapping follows that of five oil workers on Wednesday, the first since the main militant group in that area called off a month-long ceasefire.
The UK's Foreign Office called for the "immediate safe release" of the girl.
A spokesman said: "We do not know who took her. We are in contact with her parents and are providing assistance. High Commission officials are in contact with the Nigerian authorities."
The BBC's Alex Last in Lagos says that in almost all previous cases it is foreign oil workers who have been taken hostage but in recent months the children of wealthy Nigerians have also become targets.
There are a plethora of armed gangs which operate in the Niger Delta and kidnapping for ransom has become big business, he adds.
Margaret Hill was snatched by gunmen at 0730 (0630 GMT) after they smashed a window of her car as it stood in traffic.
Her mother, Oluchi, told the BBC that the kidnappers then called her at 1000 (0900 GMT) and allowed her to speak to the baby.
"She was crying... 'They took her to the bush and she wants to go to school'," she told the BBC World Service.
Mrs Hill said the kidnappers had 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, her mother said.
But Mrs Hill said she had not been contacted by the kidnappers since then.
Margaret is the third child to be kidnapped by unknown gunmen in Nigeria in the last couple of weeks.
The first girl to be taken hostage was the daughter of a Nigerian businessman. A local state legislator's daughter was also kidnapped last week.
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
Both were later released unharmed after ransom payments, the BBC's Abdullahi Kaura in Port Harcourt says.
More than 100 foreigners have been taken hostage in the region this year.
Correspondents say hostages are usually released after ransom payments that the Nigerian government and oil companies involved always deny.
Wednesday's incident involved two New Zealanders, an Australian, a Venezuelan and a Lebanese kidnapped in Soku, Rivers state.
The main militant group, the Movement for the Emancipation of the Niger Delta (Mend) said it had nothing to do with the attack.
On Tuesday, Mend announced that it would not extend its month-long ceasefire called to give the new government a chance to set up talks on the restive region.
It said it would also resume abductions and attacks on oil installations.
The group says it has been kept on the sidelines of government-led talks about the future of the Niger Delta.
Although the Delta accounts for over 90% of Nigeria's income, the region remains highly impoverished, a situation the militants say they want to change with their campaign.