UPDATE 4-Nigerian militants attack three Shell oil sites

* 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.