BP has begun critical pressure tests to see if a cap on the leaking Gulf of Mexico oil well is strong enough to halt the spill.
The cap is intended to block the oil flow until nearby relief wells are ready in August.
Oil has been gushing out of the well since a rig explosion in April that killed 11 men.
The BBC reports that testing had earlier been delayed so experts could review the procedure in the light of fears that it could irreparably harm the well bore, hindering efforts to permanently seal it.
"There is a tremendous sense of urgency," says the US Coast Guard's incident commander, Admiral Thad Allen. But nobody, he adds, wants to make an "irreversible mistake".
During the test , BP will monitor the pressure of the oil in the well.
High pressure will mean the oil has been contained inside the wellhead; low pressure may indicate that oil is leaking elsewhere in the well.
