BP's ruptured oil well in the Gulf of Mexico is expected to be permanently sealed by Sunday.
Adm Thad Allen, in charge of cleaning up the oil spill, has said a relief well, which has taken more than four months to drill, should intersect the damaged well later on Thursday.
A mixture of mud and cement will then be pumped in, permanently sealing it.
On Wednesday, the US said it would require oil firms to plug 3,500 non-producing Gulf of Mexico oil wells.
"We are within a 96-hour window of killing the well," former Coast Guard chief Adm Allen told reporters.
No oil has spilled into the Gulf since a temporary cap was placed on the well in mid-July. Mud and cement were later pushed down through the top of the well, and the cap removed.
The relief well is being drilled 2.5 miles (4km) through dirt and rock beneath the sea floor so that the ruptured well can also be sealed from the bottom, ensuring it never causes a problem again.
On Wednesday, the US government said it also wanted energy firms to dismantle hundreds of unused platforms to prevent potential leaks; some installations have been sitting idle for decades without inspection for leaks.
The new requirements are due to take effect in October.
BP's Deepwater Horizon oil rig exploded in April, killing 11 people and causing one of the worst ecological disasters in US history, with nearly five million barrels of oil spilling into the Gulf.
In London on Wednesday, BP's departing boss Tony Hayward told MPs that the oil spill was "devastating" to him.
"I understand why people feel the way they do, and there is little doubt that the inability of BP, and the industry, to intervene to seal the leak... was unacceptable."
But he said it would be wrong to blame only BP.
"No single factor caused the accident, and multiple parties including BP, Halliburton and Transocean, were involved," he told MPs on the Commons Energy Committee.