BP engineers Thursday stopped oil flowing into the Gulf of Mexico for the first time since April as they shut all valves on a new cap placed on top of a fractured wellhead, a BP official said.

"It is good to see no oil going into the Gulf of Mexico," said senior vice president Kent Wells, but he cautioned: "We are just starting the test."

He said the oil flow stopped as the last of three valves on a huge capping stack was shut at around 2:25 (1925 GMT) Thursday, but engineers were keeping a close eye on the operation to see whether any oil began leaking again.

It was the biggest step forward in halting the worst oil spill in US history which has been flowing into the sea since a BP-leased rig sank on April 22, two days after a major explosion on the deepwater drilling platform.