Pirates who hijacked a Saudi oil tanker off the Somali coast are reported to have demanded a $25m (£17m) ransom, according to BBC.

The AFP news agency, quoting one of the pirates, says the owners have been set a 10-day deadline to hand over the sum.

The Sirius Star is the biggest tanker ever hijacked, carrying a cargo of two million barrels of Saudi oil - worth more than $100m.

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The ship`s operators, Vela International, have told the BBC that they cannot comment on the reports.

On Wednesday, Saudi officials confirmed that the ship`s owners were in talks with the pirates.

AFP named the pirate it had spoken to as Mohamed Said.

"We do not want long-term discussions to resolve the matter," he was quoted as saying.

The Saudis have 10 days to comply, otherwise we will take action that could be disastrous," he added, without elaborating.

Pirates `sophisticated`

The 25 captive crew on the Sirius Star include two British citizens, two Poles, one Croatian, one Saudi national and 19 Filipinos.

The pirates who seized the tanker on Saturday are a sophisticated group with contacts in Dubai and neighbouring countries, says the BBC Somali Service`s Yusuf Garaad.

Much of their ransom money from previous hijackings has been used to buy new boats and weapons as well as develop a network across the Horn of Africa, he adds.

Russia has announced it is to send more warships to the region to patrol against pirates.

Earlier this month, one of its destroyers, the Neustrashimy (Fearless), scared away pirates who were trying to hijack ships in the Gulf of Aden.

"After the Neustrashimy, ships from other fleets of the Russian navy will head to the region," Admiral Vladimir Vysotsky said.

There has been a surge in piracy incidents off the coast of Somalia during 2008.

On Tuesday, a cargo ship and a fishing vessel became the latest to join more than 90 vessels attacked by the pirates this year.

In a rare victory against the organised gangs, the Indian navy said it had sunk a suspected pirate "mother ship".

INS Tabar attacked what was believed to be a Somali pirate "mother ship" on Tuesday after it failed to stop for investigation and instead opened fire in the Gulf of Aden, an Indian navy statement said.

The UK Foreign Secretary, David Miliband, described piracy as "a scourge wherever it appears anywhere in the world and at the moment the scourge is focused in the Gulf of Aden".

He said the Royal Navy was co-ordinating the European response to the incident.

Shipping companies are now weighing up the risks of using the short-cut route to Europe via the Gulf of Aden and Suez canal.

However, travelling around South Africa`s Cape of Good Hope would add several weeks to average journey times and substantially increase the cost of goods for consumers.