BP says the cement seal on its crippled oil well in the Gulf of Mexico is holding tight, as the company readies a final push to permanently shut down the deep-water source of the world's worst offshore petroleum spill.

The so-called "static kill" at the top of the well is due to be finished off with a "bottom kill" later in August, when more mud and cement is injected through a relief bore that will be drilled into the ruptured shaft.

This relief well is regarded as the final solution to plugging the reservoir 4,000 meters beneath the seabed.

The successful sealing was a relief for both the British energy giant, and for the US administration, which is vowing to help the pollution-stricken Gulf Coast recover. But big questions remain about just how much environmental and economic damage the oil spill has caused, and could still cause, as well as about the final costs and liabilities BP will face.