UPDATE 1-Nigerian rebels say they attacked oil tanker dock

* Rebel group claims rare attack outside Niger Delta

* Militants say they set depot, tankers ablaze

* Police investigating report, unable to confirm

(Adds details, background)

By Randy Fabi

ABUJA, July 13 (Reuters) - Nigeria's most prominent militant group said on Monday it had sabotaged a loading dock for oil tankers in Lagos state, widening an offensive against Africa's biggest oil sector.

The Movement for the Emancipation of the Niger Delta (MEND) said its fighters launched an attack on the Atlas Cove Jetty in Lagos state overnight, the first in the area since the group began its latest campaign of violence in late May.

"The depot and loading tankers moored at the facility are currently on fire," MEND said in a statement.

It was not immediately possible to independently verify the attack. A police spokesman said authorities were investigating.