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WASHINGTON (Reuters) - The flow of oil and gas from the broken well in the Gulf of Mexico has been stopped by pumping mud into it however the challenge will be whether that can be sustained, the U.S. incident commander Admiral Thad Allen said on Friday.

Allen said the next 12 to 18 hours will be “very critical” in the effort to stop the gusher which has sent thousands of barrels of oil into the fragile ecosystem there, he said on ABC’s “Good Morning America”.

BP Plc BP.L Chief Executive Tony Hayward said on the same program that the effort to plug the well was "going pretty well according to plan" and that they have also been pumping rubber and other material into the well, known as the "junk shot".

Hayward said they planned to begin pumping more mud into the well again later today.