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HOUSTON (Reuters) - Efforts to permanently plug the world’s largest offshore oil spill reached a milestone when BP Plc’s crucial relief well reached its target -- the blown-out Macondo well that began spewing oil almost five months ago, a U.S. official said.

Now that the relief well has intersected with BP’s well, workers have an opening through which they can pump in mud and cement to kill the Macondo well for good.

“The aggregate data available supports the conclusion that the two wells are joined,” retired Coast Guard Admiral Thad Allen, the top U.S. official overseeing the spill response in the Gulf of Mexico, announced late Thursday in a statement.

The next step will be to pump cement into the Macondo well near its bottom about 2 1/2 miles/ beneath the seabed, Allen said.

He also said tests indicated the reservoir already was sealed off from all parts of the Macondo well after BP pumped in cement from the top on August 5. The so-called “bottom kill” is intended to ensure the job is done.

The final kill will come more than two months after BP sealed off all flow with a cap on the wellhead on July 15. Before then, the well spewed more than 4 million barrels of oil into the sea as BP scrambled to seal it with repeated failed efforts.

The disaster began with a blowout on April 20 that sparked an explosion and fire aboard the Deepwater Horizon drilling rig, killing 11 men. The resulting oil spill fouled parts of the Gulf Coast from Louisiana east and stifled the area’s fishing and tourism industries.

Once the cement cures, BP will conduct pressure tests to ensure the well is dead, Allen said. He said when drilling resumed on the relief well early Wednesday that the entire process could take four days.