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A heavily oiled bird struggles to climb onto a boom from the waters of Barataria Bay, which are laden with oil from the Deepwater Horizon oil spill, in Barataria Bay, Louisiana. Image Credit: AP

New Orleans: BP robots have attached a new, tighter-fitting cap on top of the gushing Gulf of Mexico oil leak, raising hopes that the crude could be kept from polluting the water for the first time in nearly three months.

Placing the cap on top of the leak Monday was the climax of two days of delicate preparation work and a day of slowly lowering it into position that is 1.6 kilometres below the sea.

The capping project - akin to building an underwater Lego tower - is just a temporary fix, but the oil giant's best hope for containing the spill. Live stream: Sub sea operation

The next unknown is whether the 5.5 metre-high, 68,000kg metal stack of pipes and valves will work.

BP plans to start tests on Tuesday, gradually shutting the valves to see if the oil stops or if it starts leaking from another part of the well.