BP engineers were claiming success today after pumping heavy drilling mud into the blown-out Gulf of Mexico oil well.
Crews began the long-awaited effort dubbed the "static kill" at around 9pm yesterday.
The effort involves pumping mud and cement from ships to the well bore a mile below to seal off the source of the oil.
But the US government and oil executives will not declare victory until crews also pour mud and cement down an 18,000ft relief well later this month to help choke the vast undersea reservoir that feeds the wrecked Deepwater Horizon well -- the only way, they say, to make certain oil never escapes again.
Meanwhile, Michael Bromwich, who as director of the Bureau of Ocean Energy Management, Regulation and Enforcement is America's top drilling regulator, said the Obama administration hoped to lift a freeze on deep-water drilling before it is due to expire on November 30.
Tests for the static kill started a couple of hours earlier as BP probed the broken well bore with an oil-like liquid to determine whether there were any obstructions in the well and to assess the pressure of the bore and the pump rates it could withstand.
"This is a really positive step forward," retired US Coast Guard Admiral Thad Allen said earlier, calling it "good news in a time where that hasn't been very much good news, but it shouldn't be a cause for premature celebration".