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.