British oil giant BP had permanently "killed" the leaking Macondo well in the Gulf of Mexico, the U.S. spill commander said on Sunday.

Retired Coast Guard Adm. Thad Allen, who is tasked to oversee the spill response, said on Sunday the months-long underwater gusher "is effectively dead" and "poses no continuing threat to the Gulf of Mexico."

"With this development, which has been confirmed by the Department of the Interior's Bureau of Ocean Energy Management, we can finally announce that the Macondo 252 well is effectively dead, " said Allen.

"Additional regulatory steps will be undertaken but we can now state, definitely, that the Macondo well poses no continuing threat to the Gulf of Mexico," he said.

A pressure and weight test to ensure that a cement plug would hold was completed at 5:54 a.m. CDT, according to Allen.

BP finished pumping cement into the damaged Macondo well on Friday and had been waiting for the results of the test before officially declaring the well is plugged for good.

Comfirmation that the well is permanently sealed marks the official end of the saga that began on April 20, when an explosion on BP-leased Deepwater Horizon drilling rig killed 11 workers and unleashed the worst oil spill in U.S. history.

BP's five-month-long fight against the spill is filled with dashed hopes and blistering criticism, leading to shakeups in the government and industry and left residents along the Gulf Coast worried for their livelihoods.

The U.S. Minerals Management Service was split up into two separate agencies, one aimed at enforcing regulations and another that collected revenues. A temporary ban on deepwater drilling was announced.

Before the leaking well was sealed by a temporary cap in Mid- July, an estimated 4.9 million barrels of oil had spewed into the Gulf of Mexico, bringing tremendous harm to the Gulf ecosystem and local economies.

BP said Sunday it has paid hundreds of millions of dollars in claims for damages to Gulf Coast residents. The spill had cost the company 9.5 billion U.S. dollars as of Saturday.

Tony Hayward, the outgoing chief executive of BP, called the cementing proccess "the final step in a complex and unprecedented subsea operation."

"However, there is still more to be done. BP's commitment to complete our work and restore the damage done to the Gulf of Mexico, the Gulf coast and the livelihoods of the people across the region remains unchanged," he said in a statement.

U.S. President Barack Obama on Sunday welcomed the successful " kill" of BP's leaking Macondo well in the Gulf of Mexico, calling it an "important milestone."

Obama said in a statement that his administration remains committed to doing everything possible to ensure that the Gulf Coast recovers fully from the disaster.

"This road will not be easy, but we will continue to work closely with the people of the Gulf to rebuild their livelihoods and restore the environment that supports them," Obama said.