BANGKOK — Critics on Thursday slammed Thailand's state-owned oil and gas company for its allegedly inefficient response to an oil spill that polluted a popular tourist island, as cleanup operations continued for a sixth day.

About 50,000 liters (13,200 gallons) of crude oil — around the amount contained in 1 1/2 tanker trucks — spilled into the Gulf of Thailand off Samet Island on Saturday morning from a leak in a pipeline operated by PTT Global Chemical Plc., a subsidiary of state-owned PTT Plc.

The spill washed ashore on Samet, a small resort island that each year draws some 1 million tourists due to its pristine beaches and proximity to Bangkok, 140 kilometers (90 miles) to the northwest.

Opposition lawmaker Sathit Pitutecha said Thursday that the operator had failed to plan for such a problem despite previous leaks.

"This is a failure to solve a problem by a company that earns hundreds of billions of baht (billions of dollars) every year, using the country's natural resources," he said during a parliamentary session. "I wouldn't criticize if this was the first time, but it's the fourth time already. Now the people of Rayong (province) have to bear the burden of risk from the industrial sector."

Authorities said Thursday that the oil that blackened the waters off the island had been eliminated, but that some slicks remained onshore.

"We've worked day and night to clean up the bay," PTTGC President Boworn Vongsinudom said from Rayong. "It was supposed to be finished, as we aimed, on the third day, but more oil came in from another spot, so we had to take care of it, too."

Srisuwan Janya, head of the Thailand-based environmental group Stop Global Warming, said the company was neither prepared for the emergency nor frank about the extent of the problem.

"The transfer of crude oil through the pipeline takes place every day, but when an oil spill happens, PTTGC tackled it with unbelievably limited resources," he said. "This shows their solutions are inefficient and not enough."

The company on Sunday had flown in oil spill management experts and a plane from Singapore to get rid of the crude oil, but failed to curb the slicks from spreading.

Environmentalists fear the slow cleanup process will impact the surrounding area.

"It's plain to see that the longer the spill exists, the worse impact it would have on the environment," said Thon Thamrongnawasawat, a marine biologist at Bangkok's Kasetsart University. "But at this point, it is very difficult to estimate the damage the leak has caused to the ecosystem."

PTT is Thailand's largest energy company and one of the biggest listed oil and gas conglomerate in Asia-Pacific.

The incident is the company's fourth major oil spill in Thailand, according to the Energy Ministry.

In 2009, another PTT subsidiary was involved in the Montara oil spill, one of Australia's worst oil disasters, in the Timor Sea off western Australia.