HOUSTON, May 6 (Xinhua) -- The number of active drilling rigs in the United States increased to 705 this week, up by seven from the previous week and 257 from last year, according to the weekly data released Friday by Houston-based oilfield services company Baker Hughes.

       These active drilling rigs included 557 oil rigs operating in the U.S. oil fields, up by five from the previous week; 146 gas drilling rigs, up by two from last week; and two miscellaneous rigs, the same as last week.

       The rigs included 688 land drilling rigs, zero inland water rigs, and 17 offshore drilling rigs.

       Of them, 34 are directional drilling rigs, 646 are horizontal drilling rigs and 25 are vertical drilling rigs.

       So far, the Permian Basin in western Texas and eastern New Mexico has been the largest source of shale oil production growth in the United States, having become an engine of supply growth outside the Organization of the Petroleum Exporting Countries in the past years.