UPDATE 4-Nigeria militants attack Shell, amnesty hopes fade

* Militants say attack Shell oil well head

* Third attack since president's amnesty offer

* Militant group threatens more attacks on oil industry

(Adds Shell says investigating)

By Nick Tattersall

LAGOS, July 5 (Reuters) - Nigerian militants said on Sunday they had launched their third attack on Royal Dutch Shell (RDSa.L) since President Umaru Yar'Adua made an amnesty offer, and threatened to intensify their campaign of sabotage.

The Movement for the Emancipation of the Niger Delta (MEND) said in an emailed statement it had attacked a Shell oil well head in the Cawthorne Channel at about 0200 GMT, again dashing hopes that the amnesty offer would buy a period of calm.

"The facility connects to the Bonny loading terminal in Rivers state," it said, referring to a key oil export terminal in the eastern part of the Niger Delta.

Colonel Rabe Abubakar, spokesman for the military taskforce which patrols the Niger Delta, said "miscreants" had attacked and damaged a facility in the Cawthorne Channel, causing damage to the environment. But he had no further details.