London/Washington, Aug 4 (DPA): Oil company BP Plc said Wednesday that its so-called "static kill" operation of pumping mud into the ruptured well in the Gulf of Mexico had been successful.

The company described the development as a "significant milestone" in its attempt to place a permanent seal on the oil shaft as the well appeared to have reached a "static condition".

Pumping of heavy drilling mud into the well from vessels on the surface began late Tuesday and was stopped after about eight hours, said a statement.

"The well is now being monitored, per the agreed procedure, to ensure it remains static. Further pumping of mud may or may not be required depending on results observed during monitoring."

If the mud injection succeeds in overcoming the upward pressure of oil in the well, which stretches 5-km below the sea floor, engineers would follow by pumping in cement.

The well exploded April 20 and has become what federal scientists say is the biggest accidental oil spill in world history.

The permanent closure of the rupture can only be achieved through a parallel relief well, designed to intersect the original well shaft and allow injection of cement. This could happen as early as next week.