Transocean rig exits U.S. Gulf of Mexico

* Transocean rig under contract with Italy's ENI

* Rig left last week and headed to Nigeria

WASHINGTON, Sept 1 (Reuters) - Transocean Ltd (RIG.N: Quote) (RIGN.VX: Quote) has moved one of its deepwater rigs out of the Gulf of Mexico, as the Obama administration's deepwater drilling moratorium hampers U.S. offshore oil and gas operations.

Transocean's Marianas rig is its first to move from the Gulf since the suspension began and departed last week, bound for Nigeria, a company spokesman confirmed on Wednesday.

The rig, under contract with Italy's ENI (ENI.MI: Quote), was set to drill the Triton field.

Oil companies and Gulf state lawmakers have warned that the Interior Department's halt on exploratory deepwater drilling will push idled rigs to foreign waters and cost the region thousands of jobs.

The department imposed the six-month ban after an explosion sank Transocean's Deepwater Horizon rig, ruptured an undersea well and spilled 4.1 million barrels of oil into the Gulf.

Transocean, the world's largest drilling rig contractor, had more than a dozen deepwater rigs operating in the Gulf when the drilling ban was put in place. (Reporting by Ayesha Rascoe; Editing by Dale Hudson)