(Recasts with Exxon statement, Shell spokesman paragraph 4)

LAGOS/LONDON, April 28 (Reuters) - Nigeria's oil output was cut by around half on Monday as a strike by workers at Exxon Mobil XOM.N shut in nearly all its production and recent attacks by Niger Delta rebels hit Royal Dutch Shell RDSa.L.

Exxon said it had shut virtually all of its Nigerian output, close to 800,000 barrels per day (bpd), due to the five-day-old strike, equivalent to nearly 40 percent of output from the world’s eighth largest oil exporter.

The rebel Movement for the Emancipation of the Niger Delta (MEND) said, meanwhile, it had learned from sources at Shell that an attack on a major pipeline on Thursday had forced the company to shut-in an additional 350,000 bpd of production.

A Shell spokesmen declined to comment on how much output had been disrupted by a spate of recent attacks, but said the company was working to recover spilled oil and undertake repairs.

Shell said last week that a prior MEND bombing had obliged it to cut 169,000 bpd of crude output from its Bonny Light stream.

The problems in Nigeria helped push oil prices to a record high near $120 a barrel on Monday.

“From Friday, (Exxon workers) have been gradually shutting down operations. They have shut down 770,000 barrels per day of production, literally all of their production,” a senior oil official told Reuters in a telephone interview.

Exxon could not say how long the oil would be shut in.

“Essentially all production of ExxonMobil’s affiliates in Nigeria has been shut in. The affiliates will not speculate about the length of the shut-in nor how long it will take to restart production,” the U.S. major said in an emailed statement.

The Petroleum and Natural Gas Senior Staff Association of Nigeria (PENGASSAN) called the strike at Exxon Mobil’s local unit, targeting oil production and exports, after failing to reach agreement with the company over a labour dispute.

“There is a complete shutdown of all offices, flow stations, everything. There is no production,” one of the sources told Reuters.

TALKS TAKING PLACE

Negotiations between Exxon and PENGASSAN, brokered by the state-run Nigerian National Petroleum Corp. (NNPC), resumed on Monday in the capital Abuja. The Nigerian government had met with the feuding parties last week in an effort to avert any loss in output.

NNPC spokesman Levi Ajuonuma said both parties were eager to resolve the matter and output could be restored 48 hours after a resolution was reached.

PENGASSAN Deputy Secretary-General Lumumba Okugbawa expressed hope a deal would be sealed on Monday.

Shares in Shell, which has born the brunt of the rebel attacks in the Delta, slipped after the latest MEND statement, trading down 0.2 percent at 1447 GMT, compared to a 0.7 percent rise in the DJ Stox European oil and gas sector index.

“Our sources inside the Shell Petroleum Development Company has informed us today that the consequence of our April 24 attack on the Kula major trunk line is the disruption of a further 350,000 bpd, bringing the total shut-in so far to over 500,000 bpd,” the MEND said.

If confirmed, the supply loss would bring the amount offline in Africa’s largest oil exporter to nearly 1.7 million bpd, well over half of its potential output, according to oil companies and industry sources. (Additional reporting by Nick Tattersall, Randy Fabi and Tom Bergin in London)