HOUSTON/MIAMI (Reuters) - BP BP.L is continuing with its "top kill" operation to try to plug its leaking Gulf of Mexico oil well but has no immediate update to give on whether it has succeeded, a BP spokesman said on Thursday.

Spokesman David Nicholas was responding to a Los Angeles Times report that had quoted a U.S. Coast Guard admiral leading the oil spill response as saying the top kill procedure had succeeded in blocking the leak.

“The top kill operation continues, there are no operational updates,” Nicholas told Reuters.

He could not say when the next progress report would be given but recalled that BP executives had said it could take up to two days to tell whether the top kill operation, which started on Wednesday, had worked.

The oil spill response unified command, which includes both BP and the Coast Guard, said it could not confirm or deny the Latimes.com report, which cited Coast Guard Admiral Thad Allen.

BP shares were up more than 5 percent in London trading.

Allen told National Public Radio that BP engineers had “been able to force mud down and not allow any hydrocarbons to come up.” But he added this did not mean the well was sealed or that the problem was solved.

The top kill attempt, which has been underway for more than 20 hours, involves pumping heavy drilling fluids into the top of the well to try to halt the oil flow. If it succeeds in doing this, cement would then be injected to cap the well.

BP Managing Director Robert Dudley said earlier on Thursday the procedure was “moving the way we want it to.” But he told NBC’s “Today Show” it was too early to say whether it had been successful.