"This is not the end, but it will virtually assure us that there will be no chance of oil leaking into the environment," retired Admiral Thad Allen, who oversees the spill response for the government, said in Washington.
It could take at least a day for the cement pumped into the blown well to dry, and another five to seven days for crews to finish drilling the final 30 metres of the relief well.
Then the pumping process in the relief well could last days or even weeks, depending on whether engineers find any oil leaks, Allen added.
In recent days BP officials have refused to commit to pumping cement down the relief well, saying only that it will be used in some fashion.
They have also not elaborated on other options, but those could include using the relief well simply to test whether the reservoir is plugged.
"We have always said that we will move forward with the relief well. That will be the ultimate solution," Kent Wells, BP senior vice-president, said on Wednesday.
"We need to take each step at a time. Clearly we need to pump cement. If we do it from the top, we might alter what we do with the relief well, but the relief well is still a part of the solution. The ultimate objective is getting this well permanently sealed."
Allen reiterated on Thursday that to be safe, the gusher will have to be plugged up from two directions, with the relief well being used for the so-called "bottom kill".
"The well will not be killed until we do the bottom kill and do whatever needs to be done," he said. "I am the national incident commander and I issue the orders. This will not be done until we do the bottom kill."
The procedure involved pumping mud and cement into the blown-out well.
|The 'static kill' operation involved injecting mud and cement into the well to seal the leak
A statement released by BP on Wednesday, said: "The well pressure is now being controlled by the hydrostatic pressure of the drilling mud, which is the desired outcome of the static kill procedure carried out yesterday".
The leak was caused by an explosion on the Deepwater Horizon rig on April 20, which caused it to collapse and fracture the well head.
Eleven people were killed in the incident.
A US government report published on Wednesday said that nearly three quarters of the spilled crude had now been mopped up or dispersed.
US scientists said in the inter-agency report that burning, skimming and direct recovery had removed one quarter of the oil.
Another 25 per cent had naturally evaporated or dissolved, and 24 per cent had been dispersed, either naturally or chemically, the report said.
The rest was either on or just beneath the water's surface as "light sheen or weathered tarballs," had washed ashore or was buried in sand and sediments at the sea bottom, the scientists said.
But more than one million barrels of oil remains in the Gulf, four times the estimated 257,000 barrels that spilled into Prince William Sound from the Exxon Valdez tanker in 1989.
If BP is found guilty of negligence over the spill, the flow rate of 62,000 barrels a day means it could face up to $17.6bn in fines.
The firm has also set up a $20bn fund to pay claims from individuals and businesses hit by the disaster.
Gulf fishing communities and business owners are still counting the cost of the spill, which forced the closure of large swaths of rich fishing grounds and dealt a severe blow to local tourism.
The spill is the world's largest accidental release of oil, surpassing the 1979 Ixtoc well blowout in Mexico's Bay of Campeche that spilled out almost three million barrels.