Oil concern BP has resumed its so—called “top kill” technique to try to cap a gushing seabed oil well after a day of stopping to see whether its effort was working, the New York Times reported early on Friday.

The paper cited BP officials as saying work had resumed late Thursday night local time in the Gulf of Mexico in the effort to pump heavy drilling liquids into the well to plug the leak.

BP had begun the top kill effort late Wednesday and there were initial indications that the method might be showing the hoped—for results.

But then the company hit the pause button to give BP engineers a chance to assess the results, with chief operating officer Doug Suttles admitting that “we have not yet stopped the flow.” Mr. Suttles tried to put the best face on what many saw as a setback.

“I would characterize what we’ve done so far as part of the plan,” he said. “It’s difficult to be optimistic or pessimistic.” For most of Thursday, BP engineers had been “assessing the first results” and restocked vessels with 15,000 barrels of the heavy engineered mud for the next round of operations, he said.

BP head Tony Hayward has cautioned that top kill carries a risk of making the leak even larger, so engineers are proceeding with caution.

The halting nature of the process was necessary to allow engineers to measure the pressure in the pipes, Mr. Suttles said.

“You stop pumping. If the pressure begins to rise, you pump again.

If it doesn’t rise, you have some sign of success,” Mr. Suttles said.

In the two attempts on Wednesday, Mr. Suttles said it was hard to “be certain” whether the pressure of the pumped mud had ever countered that of the crude oil spurting up from six kilometres beneath the ocean floor.

Meanwhile the encroaching crude oil slick has heightened fears along the fertile Gulf Coast and provoked political handwringing in Washington. US President Barack Obama on Thursday ordered exploratory rigs in the Gulf of Mexico to halt operations and extended a moratorium on new permits.

Mr. Obama was scheduled to visit the stricken region on Friday, his second inspection trip since the disaster began late on the night of April 20 with an explosion on the Deepwater Horizons oil rig some 85 kilometres off the Louisiana coast. The rig sank two days later.

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