Global oil prices have risen on the back of violence and political turmoil in Nigeria, the world's eighth largest crude exporter.

With militant attacks shutting down a fifth of Nigeria's oil production, London's Brent crude was up $1.48 cents to $67.97 a barrel by Monday evening.

Meanwhile, US light crude was up $1.23 to $64.32 a barrel.

The attacks on Nigerian oil facilities came in the wake of the country's controversial presidential elections.

Umaru Yar'Adua, of the ruling People's Democratic Party, was declared the winner on Monday, but European Union observers have said the result is "not credible".

Alongside numerous reports of electoral fraud, at least 200 people have died during the campaigning, and opposition parties have vowed not to accept the result.

'Violence and sabotage'

"The situation in Nigeria may yet get worse before it gets any better," said Edward Meir, an analyst at Man Financial Energy Group.

The attacks on oil facilities recently caused Anglo-Dutch group Shell, the biggest overseas oil investor in Nigeria, to shut down its main Forcados field.

"We are looking to resume production in the next months," a Shell spokeswoman said.

But Mike Wittner, of investment bank Calyon, said he was concerned that violence against oil facilities may continue.

"If there is widespread popular disenchantment with the elections, that raises the possibility that the violence and sabotage specifically directed against the oil industry might continue," he said.

Source: news.bbc.co.uk