A remote-controlled submarine shot a chemical dispersant into a massive undersea oil leak yesterday. Crews using the deep-sea robot attempted to thin the oil – which is rushing up from the seabed at a pace of about 210,000 gallons per day – after getting approval from the Environmental Protection Agency, a spokesman for BP said. The agency had halted two previous rounds of the dispersant to test its potential impact on the environment.

After an ice build-up thwarted attempts to siphon off most of the leak using a 100-tonne containment box, BP engineers pushed ahead with other potential short-term solutions, including using a smaller box and injecting the leak with junk to plug it. However, none of these have been tried so deep – about a mile below the surface .

Workers were simultaneously drilling a relief well, the solution considered most permanent, but that was expected to take up to three months. At least 3.5 million gallons were believed to have leaked since a drilling rig blast killed 11 on 20 April. If the leak continues unabated, in about a month it will surpass the 1989 Exxon Valdez disaster as the worst US oil spill.

The engineers appear to be "trying anything people can think of" to stop the leak, said Ed Overton, a Louisiana State University professor of environmental studies.

Back on land, authorities in Louisiana deployed helicopters to drop sandbags along islands and marshes where the oil has already reached. Authorities also planned to use south Louisiana's system of locks and levees to release water to help keep the worst of the oil at sea.

BP – which is responsible for the clean-up – said the spill has cost it $350m (£235.5m) so far for its immediate response and containment efforts.