PORT HARCOURT, Nigeria (AP) ¡ª Unknown assailants attacked an oil installation in restive southern Nigeria and some crude production has been lost, Royal Dutch Shell PLC officials said Saturday.

Shell spokesman Precious Okolobo said the attackers hit a flow station belonging to Shell's joint venture in southern Nigeria late Friday and that some oil production had been shut down. He gave no further details.

Flow stations are intersections for tubes carrying oil from wells to export terminals.

The region's main militant group, the Movement for the Emancipation of the Niger Delta, did not immediately reply to an e-mail seeking comment or claims for the attack.

Militants are stepping up attacks on oil infrastructure in Africa's biggest oil producer as one of their reputed leaders is put on trial for treason and terrorism, which carry a possible death penalty for conviction. Militants say they are agitating to force the federal government to send a greater share of the country's oil revenues to their region. The southern Niger Delta where Nigeria pumps its crude remains desperately impoverished, despite four decades of oil production in the area.

But crime and militancy are closely interlinked, with gunmen stealing oil from pipelines for overseas resale one day and professing political aims for their attacks on oil infrastructure the next.