BP said the cement seal on its crippled Gulf of Mexico oil well was holding on Friday, as the company readied a final push to permanently shut down the deepwater source of the world's worst offshore spill.

More than 100 days after the start of the catastrophic spill that ravaged ecologically sensitive wetlands and lucrative coastal economies in the US Gulf, BP said no oil was leaking from the undersea Macondo well and no "recoverable oil" was left on the surface of the Gulf of Mexico.

BP finished pumping cement into the ruptured well on Thursday after injections of heavy drilling mud earlier this week subdued the upward pressure of oil and gas. The wellhead was provisionally capped in mid-July.

The so-called "static kill" at the top of the well is due to be finished off with a "bottom kill" later in August with more mud and cement injected through a relief bore that will be drilled into the ruptured well shaft. This relief well is regarded as the final solution to plug the reservoir 13,000 feet (4,000 metres) beneath the seabed.

BP's chief operating officer for exploration and production, Doug Suttles, told reporters at a briefing in New Orleans the cement job "appears to be performing as expected," though tests were continuing on Friday. "All of the indications so far look very encouraging," he said.

He added the critical relief well was expected to intercept the Macondo well shaft "right at the middle of the month."

The well's sealing was a relief for both the British energy giant, whose shares and image have taken a beating from the spill, and for President Barack Obama's administration, which has pledged to help the pollution-struck Gulf Coast recover.

However, big questions remained about just how much environmental and economic damage the spilled oil had caused and could still cause, and about the final costs and liabilities to be faced by BP.

"We're far from finished," Suttles said.

"Clearly we feel like its moving to a new phase because we've been three weeks without new oil flowing into the sea, and we don't have oil out on the open water anymore. But we still have a lot of work around the shoreline," he said.

BP shares, which have recovered strongly since hitting a 14-year low on June 25, appeared once again to be buoyed by the news of the progress on plugging the well. They rose more than 2 per cent in early London trading before falling back to a 0.46 percent gain later in the day. BP shares were up 0.64 per cent in mid-day trading in New York.

The company has lost over a third of its market value since the April 20 blast that killed 11 workers, sank the Deepwater Horizon rig and triggered the spill. – Reuters