The US Coast Guard said the source of the leak from the boat had been contained after the accident, Xinhua news agency reported.

The Coast Guard said it deployed planes and drones Thursday to assess the extent of the oil spill, while closing about 6.5 miles (10.5 km) of the Gull Intracoastal Waterway, a busy shipping channel to the area. Galveston is approximately 50 miles (80.5 km) from downtown Houston, Texas' largest city.

"We are confident that much less oil was released into the water than initially estimated," Coast Guard Captain Keith Donohue said at a news conference.

"We recovered more than 605 gallons of oily water mixture from the environment, as well as an additional 5,640 gallons of oil product from the top of the barge that did not go into the water," Donohue said.,

The 321-foot barge, which has a capacity to hold 30,000 barrels of oil, was carrying 23,000 barrels, equivalent to about 966,000 gallons, when it collided with a pillar of the Pelican Island Causeway bridge on Wednesday, Rick Freed, Vice Martin Marine, president of the barge operator, told a news conference.

A tugboat lost control of two barges "due to breakage of the coupling". The Coast Guard said a boat collided with the bridge.

Freed said the investigation is still ongoing.

The accident caused a partial collapse of the bridge, closing the only lane connection from Galveston to Pelican Island.There is no report of anyone being injured.

"The harmful consequences of oil are once again affecting our coastal communities, wildlife and waters," Joseph Gordon of Oceana, an ocean conservation group, said in a statement.

Texas Tech University professor Danny Rieble told ABC News on Thursday that the spill would probably have minimal long-term consequences, given the amount of oil on the ship.

The accident occurred just weeks after a cargo ship collided with a support column of the Francis Key Bridge in Baltimore on March 26, killing six people.