HOUSTON (Reuters) - Shares of companies that provided services or operated the sunken Deepwater Horizon drilling rig, including Halliburton Co HAL.N and Transocean Ltd RIGN.SRIG.N, fell on Friday as worry about liability from a growing oil spill in the U.S. Gulf of Mexico mounted.

But some shares rebounded or trimmed earlier losses, bouncing back after several days of big declines.

The Philadelphia Stock Exchange Index of oilfield services company .OSX, which includes Halliburton and Transocean, fell 2.3 percent in afternoon trading.

“This direct liability concern is valid, but longer term concern for disruption to activity in deepwater Gulf of Mexico and (and potentially elsewhere) is an incremental worry,” energy research firm Tudor Pickering Holt & Co said in a note to clients on Friday. “(Exploration and production) companies generally indemnify the oil service companies and drillers against well control issues.”

Transocean owned the Deepwater Horizon, which sunk last week after an explosion that likely killed 11 workers. Oil gushing from the ocean floor is heading toward the Louisiana shore, threatening wildlife and commercial fishing operations.

The rig was contracted to BP Plc BP.LBP.N, which bears the cost of clean-up from a widening oil spill. Anadarko Petroleum Corp APC.N had a 25 percent interest in the well.

Halliburton provided a variety of work on the rig, while Cameron International Corp CAM.N supplied the blowout preventer for the rig.

Shares of BP traded in the U.S. fell 0.5 percent while shares of Transocean on the New York Stock Exchange skidded over 7 percent. Halliburton was down almost 3 percent and shares of Cameron rose 2.6 percent.

Anadarko shares were down over 6 percent.