* Group warns of attacks on downstream oil sector
* Military calls for increased surveillance
* Security situation already tense ahead of election
(Adds military comment, details from court hearing)
By Nick Tattersall
LAGOS, Jan 18 (Reuters) - A Nigerian militant group on Tuesday threatened attacks on the fuel industry, telling residents to move away from storage depots and saying it would also target vehicles transporting petroleum products.
The military urged companies involved in Nigeria's downstream oil sector -- which includes refiners and fuel distribution firms -- to increase surveillance of their infrastructure and fleets following the threat.
The Movement for the Emancipation of the Niger Delta (MEND) warned in a statement emailed to news organisations of a "ferocious attack" in response to "the handling of the arrest and detention of our respected brothers".
The group's suspected mastermind, Henry Okah, is being held in South Africa while Okah's brother Charles and three others face terrorism and treason charges at home in connection with car bomb attacks in the capital Abuja last October.
"Advance warning for immediate evacuation is hereby issued to residents in close proximity to depots storing petroleum products such as aviation fuel, diesel, kerosene, petrol, propane gas and engine oil," the MEND statement said.
"Anyone who chooses to ignore this warning does so at their own peril," it said.
With its state-owned oil refineries in disrepair, Nigeria relies heavily on imported fuel for its energy needs and a campaign of attacks on fuel depots could undermine government efforts to end fuel shortages and create popular resentment.
There has already been violence in some parts of Nigeria ahead of a presidential election on April 9 and attacks by Niger Delta militants could be an embarrassment for President Goodluck Jonathan, the first head of state from the region.
His supporters believe some of the unrest has been deliberately orchestrated to discredit him.
MEND was for years the main militant group in the Niger Delta, blowing up pipelines and other installations in Africa's biggest oil and gas industry in what it said was a struggle for a greater local share of the natural wealth.
Many of its known commanders accepted the amnesty brokered by Jonathan a year-and-a-half ago, but splinter factions under newly emerging leaders have continued to use its name, claiming responsibility for car bombings in the capital Abuja on Oct. 1.
Not all of its threats have been carried out, but the latest statement was sent from the same email address as previous warnings and signed with the same pseudonym, Jomo Gbomo.
"If MEND or whatever is left of it has any iota of sanity, then it should wait for the law to run its course on its arrested members," military spokesman Timothy Antigha said.
The Abuja car bombs near an independence day parade last October killed at least 10 people and marked the group's first attack of its kind at such a high-profile public event.
Henry Okah was charged shortly afterwards in Johannesburg, where he lives, with conspiracy and terrorism, which he denies.
His brother Charles, whom the secret service says was one of the users of the "Jomo Gbomo" pseudonym, and three others were arrested in Lagos and were last month denied bail.
Charles Okah collapsed in court in Abuja on Tuesday and was stretchered away, leading the judge to suspend until Jan. 27 a hearing on whether or not he is fit to stand trial.
Okah's lawyer Ogheneovo Otemu told reporters his client and the co-accused were being kept in isolated detention.
"All other inmates have time for exercise, they go out to play, but this particular set of inmates are kept in a solitary cell and that has affected their state of health," he said. (For more Reuters Africa coverage and to have your say on the top issues, visit: af.reuters.com/) (Additional reporting by Samuel Tife in Yenagoa and Camillus Eboh in Abuja; Editing by Louise Ireland)