NEW ORLEANS – Dozens of cargo ships, petrochemical tankers and smaller vessels stacked up Thursday near a closed stretch of the Mississippi River, a day after a collision between a barge and tanker spilled more than 400,000 gallons of fuel oil into the nation's busy shipping waterway.

The Coast Guard said reopening nearly 100 miles of river to ship traffic could take days, and efforts to clean up the oily sheen left by largest oil spill on the river since 2000 could take weeks. Many of the ships waited at the river's Gulf of Mexico outlet to head upriver to grain and petrochemical terminals above New Orleans, one of the world's busiest ports.

Only about 6,900 gallons of oil had been cleaned from the fast-flowing river by midday Thursday, a fraction of the 419,000 gallons stored aboard the barge that split open early Wednesday in the collision with the Liberian-flagged tanker Tintomara.

The Coast Guard said 38 southbound vessels and 21 northbound vessels had been halted, a stackup expected to grow. Grain barges moving south from the American heartland were able to reach huge elevators at the Port of South Louisiana – upriver from New Orleans – but grain cargo ships were unable to continue out through the Gulf of Mexico.

A cruise ship scheduled to dock in New Orleans on Friday night probably will be unable to do so, said Coast Guard Capt. Lincoln Stroh, captain of the port of New Orleans. The 2,056-passenger Fantasy is readying contingency plans to dock in nearby Mobile, Ala., if needed, said Tim Gallagher, spokesman for Carnival Cruise Lines.

Meanwhile, it was unclear how the ship stoppage would impact the flow of refined products from the 10 petroleum plants that line the river between New Orleans and Baton Rouge, La. Calls to refinery officials seeking comment were not immediately returned.

But imports of crude oil did not appear to be affected. About 15 percent of U.S. oil imports come through the Louisiana Offshore Oil Port along the coast – the only U.S. port capable of handling the largest oil tankers. The complex is linked by pipeline to refineries.

Meanwhile, John Hyatt, vice president of Irwin Brown Co., a New Orleans-based freight forwarder, said it was too early to estimate the overall costs of the shutdown. But he said he expected the tab would quickly climb into the millions of dollars.

Paul Book, vice president of American Commercial Lines Inc. of Jeffersonville, Ind., which owns the barge, said about 350 cleanup workers deployed using 45 boats. They initially set out about 50,000 feet of containment booms to collect the fuel oil and planned to lay down another 30,000 feet while some used vacuum skimmers.

“This is a very large, very fast-moving river. It makes the job very difficult to contain the oil,” said Charlie Henry, scientific coordinator for the National Oceanic and Atmospheric Administration.

State authorities were optimistic environmental damage could be contained, reporting no problems from air quality tests as wildlife officials reported spotting only a few ducks and one egret coated with oil.

The fuel oil was expected to join farm chemical runoff and other pollutants dumped in along the Mississippi's 2,300-mile length and its tributaries.

The oil spill was the largest on the river since November 2000, when a tanker ran aground about 40 miles south of New Orleans, spilling more than half a million gallons of crude oil on the Mississippi. That spill closed about 26 miles of the river.

At the Port of New Orleans, spokesman Chris Bonura said the port stands to lose about $100,000 in fee revenues each day the river stretch remains closed.

At the Port of South Louisiana, spanning 54 river miles between New Orleans and Baton Rouge, work was limited to unloading ships already in port, said spokesman Lester Millet.

Michael Lorino, president of the Associated Branch Pilots, which guides ships from the Gulf into the Mississippi mouth, reported a backlog of 25 ships in the Gulf and said the line could grow by as many as 20 a day.

Meanwhile, authorities said they were investigating why the tugboat towing the barge did not have a properly licensed pilot, the Coast Guard said. Coast Guard Lt. Cmdr. Cheri Ben-Iesau said the person operating the tug, who was not identified, had an apprentice mate's license.

Associated Press writers Michael Kunzelman, Becky Bohrer and Cain Burdeau in New Orleans contributed to this report.