Hundreds of gallons of fuel spilled from an oil tanker Friday, creating a slick on the San Francisco Bay but not making it to shore, the mayor said.

"They're doing an admirable job containing the spill," said Mayor Gavin Newsom, adding that it was still unclear exactly how much oil escaped into the bay.

"Environmental concerns are the primary focus of our efforts right now," Newsom said.

The U.S. Coast Guard responded quickly after a fuel line ruptured during a fuel transfer on the Panamanian-flagged Dubai Star, about 2 1/2 miles southeast of the San Francisco-Oakland Bay Bridge. The Coast Guard says an initial investigation reveals the failure was mechanical.

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More than 1,100 feet of boom have been deployed around the ship to contain the oil, which created a slick about 2 miles long. Additional barriers were laid out to keep the oil from drifting toward Treasure Island and Yerba Buena Island.

Coast Guard and the California Department of Fish and Game are working to contain the fuel, and a nonprofit specializing in oil-spill cleanup is helping. There are so far no reports of oiled wildlife.

Newsom said officials learned from the 2007 Cosco Busan oil spill, which leaked 50,000 gallons of fuel into the bay, killed about 2,400 birds and cost $70 million to clean up.

Staff from the San Francisco Bay Regional Water Quality Control Board are monitoring the spill's potential impact on the environment, and ensuring that any damage is mitigated, said Executive Officer Bruce Wolfe in a statement.

The Regional Water Board will also be responsible for assessing any fines that might result from the spill and from any violations of state and federal clean water laws.

San Francisco officials are advising people to avoid contact with water in the Bay and refrain from fishing.