UPDATE 1-Some Shell Nigeria output shut in on attack report

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LAGOS, June 29 (Reuters) - Royal Dutch Shell (RDSa.L) said on Monday it had shut in some oil production at its western operations in Nigeria's Niger Delta while it investigated reports of attacks on two well clusters in its Estuary Field.

"We have received reports of attacks on two well clusters in the Estuary Field in the western operations. We have shut in some production as a precautionary measure while investigations progress," a Shell spokesman in Nigeria said.

Nigeria's main militant group, the Movement for the Emancipation of the Niger Delta (MEND), said earlier its fighters had attacked an oil platform in Shell's Forcados fields in the southern state of Delta.

Shell said the attacks being investigated were not in Forcados itself but in its western operations, all of which feed into the Forcados oil export terminal.

Forcados is one of Nigeria's benchmark crude oil grades but production has been subject to delays for months due to militant attacks on oil facilities and disruptions to production.

Shell extended a force majeure on June 17 on its Forcados oil shipments for the rest of June and all of July. The measure, which frees it from contractual obligations, was first imposed in March after an attack on its trans-Escravos pipeline.

(For full Reuters Africa coverage and to have your say on the top issues, visit: af.reuters.com/ ) (Reporting by Nick Tattersall)