However, BP officials did not confirm the success and the White House called the report premature. BP said an update on flow rates from the leaking riser will be provided by the U.S. Geological Survey this morning.
Allen, who is tasked with coordinating the government response to the oil spill, said engineers had succeeded in stopping the flow of oil and gas into the Gulf of Mexico from the BP well by pumping drilling mud through the inoperable blow-out preventer.
One danger of the top kill is that other leaks could erupt from the damaged pipeworks.
"We're doing everything we can to bring it to closure, and actually we're executing this top kill job as efficiently and effectively as we can," BP Chief Operating Officer Doug Suttles was quoted as saying.
Additional leaks springing from the top kill solution were a grave risk, said Anil Kulkarni, a mechanical engineering professor at Penn State.
"One scenario is that it may make things worse," Penn State mechanical engineering professor Anil Kulkarni was quoted as saying. "If it ruptures all over, then it would be even more difficult to close it."