Deepsea robots in use for three decades in oil exploration are built for demanding tasks, but perhaps none as daunting as the effort to fix the ruptured Gulf of Mexico pipeline, experts said Thursday.

The precision hands of underwater robots successfully sliced through an underwater wellpipe using hydraulic shears and are now set to undertake the task of placing a containment cap over the leak.

The move came Thursday after a diamond-blade saw got stuck Wednesday in the pipe lying a mile (1,600 meters) down on the sea bed, highlighting the complexity of the operation.

"What they're doing now is something kind of hard to train for," said Wisley Kirkland, an official at the firm Seatrepid, based in Robert, Louisiana, which frequently makes use of such robots.

He said there has been an improvisational aspect to the current operation and officials have been forced come up with a variety of strategies to try to repair the ruptured hardware.

"This is the first time I've heard of someone trying to use a diamond saw on a cutoff when it's actually producing. Usually you cut it off" first, said Kirkland, who called the task confronting BP officials "daunting."

"Those guys are doing everything they can. What they're doing is not a walk in the park," he said.

Oil giant BP has tried and failed several times in the past six weeks to cap the leak, triggering mounting anger as oil washes up on the Gulf shores, threatening rare animal and plant life, as well as local livelihoods.

As challenging as the repair job is, the work would be altogether impossible without the submersible robots or "remotely operated underwater vehicles" (ROVs).

ROVs, unoccupied and highly maneuverable underwater robots operated by a person aboard a surface vessel, are tasked with maintenance and repair work at depths too great for human divers. But Kirkland said the depth was not the biggest obstacle.

"The deeper you go, the less currents you have to deal with," he said.

But he added "it takes longer to get down there, so if there is an issue (such as) mechanical complications it takes a lot longer to get down with assistance and fix it."

He added that "as the actual operation goes, it's the same."

The deepsea robots are designed to help with the process of extracting the vast reservoirs of oil and gas from below the Gulf of Mexico floor and to cope with the currents and variable topography that makes drilling and extracting a mile underwater so challenging.

On its website, BP explained the robot arms have become more precise over decades of use.

"An ROV's arms, or 'manipulators'... have been developed over the years to have the same range of motion as a human arm. In fact, some are so sophisticated that, theoretically, they could pick up an egg without breaking it," the company says on its site.

The devices are linked by cable to ships and manipulated at a distance via a joystick, a little like those used with video games.

Outfitted with cameras and projectors, moved by propellers, each robot generally needs about three operators, Kirkland said.

"The pilot is the person controlling the ROV itself, up, down, forward, left," he said.

"The manipulator is the person who's actually operating the arms, and the navigator sits back, watches the sonar screen and takes notes and records of what's going on," he said.

The robots, linked to the ship by a group of cables that carry electrical signals back and forth between the operator and the vehicle, are usually equipped with at least one video camera and lights, and sometimes come with other instruments as well depending on the task at hand, such as a still camera or cutting arm.

It has been 45 days since an explosion ripped through BP's Deepwater Horizon rig, killing 11 workers and setting in motion what US officials are now calling the worst environmental disaster in US history.

Two deep relief wells BP is drilling into the seabed to plug the leak permanently will not be ready until August.