NEW ORLEANS — The Mississippi River is closed Thursday to ship traffic from New Orleans to the Gulf of Mexico after a barge was split in half in a collision with a tanker, spilling thousands of gallons of heavy oil.

The Coast Guard says Wednesday's accident occurred as a tugboat without a properly licensed pilot was pushing the barge.

An investigation by the Coast Guard and the National Transportation Safety Board is under way.

It was unclear how long the river would be closed to ships.

By dawn Thursday, the Coast Guard says an oil sheen extends from New Orleans almost all the way to the Gulf of Mexico about 97 miles away. About 45,000 feet of containment boom has been placed on the river and workers plan to put down another 45,000 feet and use vacuum skimmers to pick up the oil.