HOUSTON, June 17 (Xinhua) -- The number of active drilling rigs in the United States was 740 this week, up by seven from the previous week and by 270 from last year, according to the weekly data released Friday by Houston-based oilfield services company Baker Hughes.

These active drilling rigs included 584 oil rigs operating in U.S. oil fields, up by four from the previous week; 154 gas drilling rigs, up by three from last week; and two miscellaneous rigs, the same as last week.

The rigs included 721 land drilling rigs, three inland water rigs, and 16 offshore drilling rigs.

Of them, 39 were directional drilling rigs, 674 were horizontal drilling rigs and 27 were 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.