NEW ORLEANS (AP) — Several dozen ships and hundreds of barges are waiting to be cleaned before they can leave the area where a massive oil spill closed down part of the Mississippi River at New Orleans, a Coast Guard official said Sunday.

By Sunday morning, 18 vessels stranded by Wednesday’s spill were cleaned and allowed to travel to their destinations. The Coast Guard also allowed an additional 10 ships to enter the river from the Gulf of Mexico and five ships go from the river north of the spill to the Gulf.

Coast Guard Capt. Lincoln Stroh estimated it will take at least two days to clean 50 to 60 other ships and several hundred barges that have been anchored since the spill.

‘‘It’s picking up as we speak,’’ Stroh said. ‘‘Our ability to clean has drastically improved with each vessel.’’

More than 400,000 gallons of fuel oil spilled when a tanker collided with a barge early Wednesday. The spill shut down a 100-mile stretch of the river.

Cleaning up the spill is expected to take weeks. By Sunday, crews had recovered about 55,000 gallons of oil mixed with water.

‘‘The conditions are improving. Each day looks a little bit better,’’ said Paul Book, vice president of American Commercial Lines Inc. of Jeffersonville, Ind., which owns the barge involved in Wednesday’s collision.