ABUJA, Aug 8 (Reuters) - Nigerian ransom seekers kidnapped the 11-year-old son of a state legislator in the southern oil producing Niger Delta, the third child abduction in just over a month, authorities said on Wednesday.

Kidnappings for money have become increasingly common in the delta, although children were rarely targeted until recent weeks.

"They broke into the house through the roof and got into the room of the 11-year-old child. The parents have received calls from the kidnappers," said a Bayelsa state government official. He said the mother was a member of the state house of assembly.

Abductions of relatives of prominent people are a new trend in the delta. The elderly mother of the speaker of the Bayelsa house of assembly was released on Aug. 3 after 10 days in captivity.

A private security contractor working for an oil company in the region said the boy's kidnappers had demanded a 20 million naira ($157,500) ransom. This could not be independently confirmed.

Violence escalated in the delta in early 2006, when armed groups demanding control over oil revenues and an end to neglect by politicians started blowing up pipelines and abducting foreign oil workers.

Their attacks shut down a fifth of oil output from Nigeria, the world's eighth-biggest exporter, contributing to record high oil prices on international markets.

But the violence spiralled out of control, with numerous armed gangs seizing the opportunity to take hostages for money, carry out armed robberies or smuggle stolen crude oil.

The commercialisation of hostage taking has been widely condemned in Nigeria -- especially the new trend of taking children or other relatives of prominent people.

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