* Militants say attack three Shell sites in Niger Delta

* Shell says investigating

* Military says attacks targeting isolated facilities

(Adds detail from militants on third strike, paragraph 3)

By Nick Tattersall

LAGOS, June 21 (Reuters) - Nigeria's main militant group said on Sunday it had attacked three oil installations belonging to Royal Dutch Shell RDSa.L in the Niger Delta, widening a month-old offensive against Africa's biggest energy industry.

The Movement for the Emancipation of the Niger Delta (MEND) said in an emailed statement it had attacked Shell pipelines at Adamakiri and Kula, both in Rivers state in the eastern Niger Delta, in the early hours of Sunday morning.

It said it had also attacked the Afremo offshore oilfields, which it believed were operated by Shell, and which it said were 14 miles (23 km) from an export terminal through which crude oil from Shell’s Forcados fields is pumped.

Shell said it was investigating reports of attacks against its installations at three locations and was carrying out fly-overs to try to assess any impact on output or the extent of any environmental damage from potential spillage.

A senior industry source said the third attack was not thought to have been on a deepwater installation, but on a facility located in or close to the mangrove creeks, where pipelines and equipment run across broad stretches of water.

The attacks are the first to strike Rivers state, the easternmost of the three main states in the Niger Delta, since the militants launched their latest campaign of sabotage following a military offensive in the western delta last month.

Persistent attacks by MEND over the past three years have cut oil output in the OPEC member, the world’s eighth biggest crude oil exporter, to less than two thirds of its installed capacity of 3 million barrels per day (bpd).

ISOLATED LOCATIONS

Industry and security experts say it is virtually impossible to prevent opportunistic attacks on hundreds of kilometres of pipeline and equipment in the remote mangrove creeks of the Niger Delta, one of the world’s biggest wetlands.

“The militants are going about attacking pipelines in isolated parts of the creeks where they know they will not encounter resistance,” said Colonel Rabe Abubakar, spokesman for the joint military taskforce in the Niger Delta.

MEND first burst onto the scene in late 2005, knocking out more than a quarter of Nigeria’s oil output -- then around 2.4 million bpd -- in a matter of weeks.

But it has largely failed to carry out such spectacular attacks since then, although the latest campaign has nibbled further at production levels in a country that relies on oil for around 90 percent of its foreign earnings.

Agip ENI.MI said on Friday a pipeline attack in Bayelsa state had halted production of around 33,000 barrels of oil and 2 million cubic metres of gas per day. [ID:nLJ121769]

Shell said on Thursday some oil production had been halted following an attack on the Trans Ramos pipeline late on Wednesday at Aghoro-2 community in Bayelsa. [ID:nLI310768]

Chevron CVX.N shut down its operations around Delta state after MEND's first attack in its latest campaign on May 24, halting around 100,000 barrels per day (bpd). [ID:nLP693007]

MEND has dubbed its offensive “Hurricane Piper Alpha” after the North Sea oil platform that blew up in July 1998 and was the worst offshore oil disaster, and warned that it might attack deepwater facilities off the Nigerian coast.

Security sources say some oil firms have been removing non-essential personnel from some offshore sites.

MEND says it is fighting against the militarisation of the Niger Delta and for a fairer share of the wealth for local villagers. But the leaders of armed gangs it works with have grown rich from a lucrative trade in stolen oil and from ransoms paid for hundreds of oil workers kidnapped in recent years.

For an analysis on the Niger Delta, click on [ID:nLI6064]

For a factbox on the insecurity, click on [ID:nLI32684] (Editing by Lin Noueihed and Michael Roddy)