WRAPUP 3-Nigerian gunmen attack Lagos dock, top rebel freed

* Main rebel group claims rare attack outside Niger Delta

* Govt frees suspected MEND leader Henry Okah

* Militants say open to peace talks with gov't

(Adds MEND comment para 12)

By Tume Ahemba and Shuaibu Mohammed

LAGOS/JOS, Nigeria, July 13 (Reuters) - Nigeria's main militant group widened its offensive against Africa's biggest oil sector on Monday despite the release of its suspected leader, raising concern there might be further attacks.

The Movement for the Emancipation of the Niger Delta (MEND) sabotaged an oil dock in Lagos state, killing five people in the group's first attack outside the Niger Delta since it began its latest campaign.

The expansion of MEND's range has heightened security concerns beyond the oil industry and dashed hopes that the release of its suspected leader, Henry Okah, would halt the militant offensive, analysts said.

"That will not put an end to militancy because there are many Henry Okahs there, except that they are not as well known," said Abubakar Momoh from Lagos State University.

MEND said its fighters set the depot ablaze overnight at the Atlas Cove Jetty in Lagos, where ships unload imported gasoline, diesel and other products.

"We were taken by surprise because the facility is well secured," Mohammed Barkindo, managing director of state-run oil firm NNPC, told reporters at the damaged dock. "This is the first time this type of incident has happened."

BOLDER ATTACKS

MEND has rarely attacked sites outside the Niger Delta, focusing mainly on oil facilities in 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 lookout for more attacks. They could happen anywhere now."

Analysts say the militant group may be trying to strengthen its position before possible talks with the government.