VENICE, La., May 27 (Reuters) - BP (BP.L)'s "top kill" operation to try to plug its leaking Gulf of Mexico oil well is still ongoing, U.S. Coast Guard Admiral Thad Allen said on Thursday. Stocks | Bonds | Global Markets "They're still pumping mud into the wellbore and they're still monitoring it ... it is ongoing," Allen, who is leading the oil spill response operation, told reporters in Venice, Louisiana.