BP has been dealt a setback to capping a massive oil spill in the Gulf of Mexico after a containment dome encountered flammable formations as it was lowered onto the leak site.

The gas hydrates, similar to ice crystals, formed on the inside of the 100-ton chamber as it neared the seabed nearly a mile below the surface, making it too buoyant and clogging it up.

Workers have moved the concrete and steel box some 650 feet to the side on the seabed while they evaluate their options.

An estimated 210,000 gallons of oil have been gushing every day from a pipe ruptured when the BP-leased Deepwater Horizon sank on April 22, two days after an explosion that killed 11 workers.

The high-stakes attempt to cap the leak, rife with expectations because it will take three months to drill relief wells to stem the flow, had been considered the best short-term solution to stave off the biggest US environmental disaster since the 1989 Exxon Valdez spill in Alaska.