HOUSTON - A portion of the busy Houston Ship Channel was shut Monday after two 600-foot ships collided in fog, causing a leak of flammable liquid.

Methyl tertiary-butyl ether, or MTBE, a gasoline additive aboard the Danish-flagged chemical tanker Carla Maersk, leaked from tanks that ruptured in the vessel's collision with a 623-foot Liberian bulk carrier, Conti Peridot, Coast Guard Petty Officer Manda Emery said.

Three cargo tanks on the vessel were ruptured, releasing an unknown quantity of the gasoline additive, said Coast Guard Capt. Brian Penoyer, commander of the Houston-Galveston Coast Guard District.

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"This is not a cargo chemical that is easy to clean up," he said.

Crews were examining the vessel's tanks to determine how much of the product may have been spilled into the 50-mile channel that connects the Gulf of Mexico to the Port of Houston.

The leak was halted about 90 minutes after the Coast Guard received word of the collision about 12:40 p.m., Emery said.

No injuries were reported but officials asked that the about 350 people who live in the area, known as Morgan's Point, remain inside their homes. They also halted activity at the Barbours Cut Terminal, where cargo vessels are loaded and unloaded.

The federal Environmental Protection Agency describes MTBE as a volatile, flammable and colorless liquid that dissolved rather easily in water. It's used to raise the oxygen content of gasoline.

Emery said it was too soon to blame rain and fog for the collision.

"We'll have a very long marine casualty investigation to follow this," she said.

The Port of Houston, a major part of the ship channel, is home to the nation's largest and one of the world's largest petrochemical complexes. It typically handles about 70 ships per day, plus 300 to 400 tugboats and barges, and consistently ranks first in the nation in foreign waterborne tonnage, U.S. imports and U.S. export tonnage. It is second in the U.S. in total tonnage.

Officials with two states agencies, the General Land Office and the Texas Commission on Environmental Quality, said they would play a backup role in any cleanup effort, deferring to the Coast Guard for any assistance they may need to provide.

The land office was providing personnel and equipment, spokesman Jim Suydam said. TCEQ could assist with water sampling and monitoring of air quality if the Coast Guard requested it, spokesman Terry Clawson said.

Records show the Conti Peridot was built in 2011 and left Panama Feb. 27 for Houston. It previously had been to Shanghai, China. The Carla Maersk, built in 1999, left Venezuela Feb. 7, arrived in Houston last Wednesday and was headed back to Venezuela.

It's the second ship collision in the channel in less than a week. No pollution and no injuries were reported last Thursday when a 445-foot tanker and 892-foot container ship bumped about 15 miles up the channel from Galveston.