Oil and gas drilling activity is still falling, and it's unclear when it will rebound, Halliburton Co. (HAL) Chief Executive David Lesar said Monday.

"There can be no certainty about when the decline in activity will bottom out," Lesar said in an earnings conference call with investors.

The rapid decline in activity, a result of low oil and gas prices and tightening credit markets, is "worse than previous cycles in terms of the speed of the decline," he said.

Most of the decline has occurred in North America, as international operations tend to react more slowly to economic cycles. Latin America will continue to be a "bright spot," with operations in Brazil and Mexico maintaining high levels of activity, Lesar said.

The largest decline came in the Permian Basin, Rockies and the mid-continental U.S.; but the fall-off was "less severe" in complex shale plays and the Gulf of Mexico, which includes deepwater operations, Lesar said. Those areas have helped in "moderating the effect" of the current environment, he said.

Oil and gas producers are "focusing their efforts in reducing service cost inflation" by renegotiating contracts, Lesar said.

The company aims to maintain its $1.8 billion capital budget program, the same amount it spent last year. "We continue to believe in the long-term prospects of this business," he said.

-By Angel Gonzalez, Dow Jones Newswires, 713-547-9214;angel.gonzalez@dowjones.com