The lure of higher oil and gas prices has pushed the number of rigs exploring for oil and natural gas in the United States to a 10-year high, stretching the drilling industry close to its limits. The domestic rig count reached 1,163 during the week ending March 23 - the highest number since December 10, 1990 - when 1,179 rigs were in use, according to oil services firm Baker Hughes Inc. Spurred by record gas prices and the biggest oil market boom for two decades, independent U.S. energy companies are putting their record revenues back into extra drilling, but are finding it increasingly difficult to staff the growing number of rigs. Unlike 10 years ago, exploration for natural gas has eclipsed that for oil, and analysts say the increase in rig use this year has not yet peaked.