UPDATE 4-Nigeria rebels attack Lagos oil dock, 5 dead

* Main rebel group claims rare attack outside Niger Delta

* Militants say they set Lagos depot, tankers ablaze

* At least five people dead, operations at jetty shut down

(Adds details of attack, deaths, reaction)

By Hannington Osodo and Tume Ahemba

LAGOS, July 13 (Reuters) - Nigeria's main rebel group said on Monday it sabotaged an oil tanker loading dock in Lagos state, killing five people in the first attack outside the Niger Delta since the group began its latest spate of violence.

The attack comes as the government prepares to release Henry Okah, the suspected leader of the Movement for the Emancipation of the Niger Delta (MEND), after more than a year in detention for suspected arms dealing.

MEND said its fighters set loading tankers and the depot ablaze overnight at the Atlas Cove Jetty in Lagos, a key port where vessels offload gasoline, diesel and other products from refineries in the southeast of Africa's biggest oil producer.

"Heavily armed MEND fighters (on Sunday) ... carried out an unprecedented attack on the Atlas Cove Jetty in Lagos state," the militant group said in a statement.

The fire killed at least five people and damaged pipelines and the terminal, which was shut down for repairs, said Capt. Geoffrey Boukoru, who led rescue efforts.

MEND has rarely attacked sites outside the Niger Delta, focusing mainly on oil facilities in the Delta, Bayelsa and Rivers states in southern Nigeria.

"They are obviously getting much bolder," said a private security source working for the oil industry. "We are on the look out for more attacks. They could happen anywhere now."

The militant group has claimed a series of attacks against the oil industry following the military's largest offensive in the Niger Delta for years in late May.

The violence has forced Royal Dutch Shell (RDSa.L), U.S. oil company Chevron (CVX.N) and Italy's Agip (ENI.MI) to shut down around 300,000 barrels per day of production in the last seven weeks, lifting global oil prices.

REBEL LEADER