PORT HARCOURT, Nigeria, May 16 (Reuters) - Gunmen who hijacked a supply ship working for U.S. energy giant Chevron <CVX.N> in Nigeria have drastically reduced their ransom demand to 5 million naira ($42,423) from 30 million naira, security sources said on Friday.

The unidentified gunmen seized the MV Lourdes Tide and 11 crew members -- one Portuguese, a Ukrainian and 9 Nigerians -- on Tuesday, the latest in a series of attacks and sabotage in the world's eighth-biggest oil exporter.

"The hijackers are hungry sea pirates. They are now asking for 5 million naira, so the pressure is on them. Let's see what happens by the end of the day, we are hopeful of their release," one security source told Reuters, asking not to be named.

The ship, operated by oil services firm Tidewater Nigeria Ltd, was carrying supplies from Onne in Rivers state to Escravos in neighbouring Delta state in the Niger Delta region when it was boarded by the gunmen.

A spokesman for a military taskforce in charge of security in the delta, the heartland of Africa's hydrocarbons industry, could not immediately confirm the ransom slash. A spokeswoman for Chevron in Houston also had no update on the hijack.

Kidnappings for ransom are common in the Niger Delta. Militant groups launched a violent campaign in 2006 to press for greater local control of oil revenues but the line between criminality and militancy has become blurred.

Oil companies and trading sources say the recent spate of attacks and sabotage have shut in about 559,000 barrels per day of Nigerian production, about 19 percent of the installed output capacity of around 3 million bpd in the West African state. (For full Reuters Africa coverage and to have your say on the top issues, visit: http://africa.reuters.com/) (Reporting by Austin Ekeinde and Tume Ahemba, Writing by Tume Ahemba, Editing by Nick Tattersall/James Jukwey)

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