* Oil firm says armed men boarded offshore facility
* Militant group says seven Nigerian workers kidnapped
* More attacks threatened (Adds statement from militant group)
LAGOS, Nov 16 (Reuters) - Nigeria's main militant group said on Tuesday it was behind an attack on an Exxon Mobil (XOM.N) oil platform and it had kidnapped seven Nigerian workers, the second such raid for which it has claimed responsibility in a week.
The U.S. energy firm said armed men boarded the offshore facility, operated by its Mobil Producing Nigeria unit in a joint venture with the state-run Nigerian National Petroleum Corporation (NNPC), late on Sunday but gave no further details.
The Movement for the Emancipation of the Niger Delta (MEND) said it had detonated explosives rigged to the facility and kidnapped seven local workers. There was no independent confirmation of any damage to the platform.
The group warned of further attacks in the heartland of Africa's biggest oil and gas industry.
"In the coming weeks, the Movement for the Emancipation of the Niger Delta will launch a major operation that will simultaneously affect oil facilities across the Niger Delta," it said in a statement emailed to the media.
Previous campaigns by MEND fighters have knocked out a significant proportion of the OPEC member's oil production, currently averaging about 2.2 million barrels per day (bpd), and cost it as much as $1 billion a month in lost revenues.
Renewed unrest in the region would be an embarrassment for President Goodluck Jonathan in the run-up to April elections. Jonathan is the first head of state from the Niger Delta and brokered an amnesty with militants more than a year ago.
The Exxon attack took place a week after a similar raid on a rig operated by the exploration firm Afren (AFRE.L) in the same waters off the state of Akwa Ibom, in which two Americans, two Frenchmen, two Indonesians and a Canadian were kidnapped.
The military threatened on Saturday to raid the camps of armed gangs in the creeks of the Niger Delta, warning civilians to leave before what could be its first major offensive in the region since a government amnesty programme began last August.
MEND said one military operation had already been carried out against one of its camps in Rivers state on Monday, adding clashes would endanger the lives of the hostages it was holding.
"Expatriate hostages held at this location had to be removed and relocated for their safety as rocket attacks by the Nigerian military came very close," the group said.
There was no immediate confirmation from the military of any operation in Rivers state.
A security source said Sunday's attack was took placed at a platform in the Oso field, one of Nigeria's biggest condensate fields with about eight platforms whose total output averages about 75,000 bpd.
"Relevant government and security agencies have been informed and appropriate response measures are under way," Exxon said in a statement, without commenting on the potential effect on output.
Oil firms in the Niger Delta suffered years of attacks until the amnesty programme bought more than a year of relative peace.
The militants were always divided, and although many of the field commanders agreed to lay down their weapons, new leaders have started to emerge, security experts say.
Besides the attack on Afren, MEND claimed responsibility for two car bombings in Abuja which killed at least 10 people near an independence day parade on Oct. 1. It was the group's first attack in the capital. (For more Reuters Africa coverage and to have your say on the top issues, visit: af.reuters.com/ ) (Editing by Andrew Dobbie)