WASHINGTON (AFP) – A long-standing deadline for sealing the ruptured Gulf of Mexico well deep below the seabed will be missed as US officials and BP tackle concerns about debris lodged in the well.

BP and US government representatives had hoped to complete a "bottom kill" procedure and officially pronounce the well dead by mid-August, but the US pointman on the oil spill response said Monday the bid was on hold.

Admiral Thad Allen said that an earlier successful process sealing the well from above may have wedged cement between two layers of casing, trapping leaked crude inside the void.

That has forced officials to put on hold plans to seal the well from below by flushing first a heavy drilling fluid called "mud" and then cement into the damaged structure via an intercept from a second relief well.

"When we do the intercept well and commence pumping mud and cement... it will create pressure," Allen said.