WASHINGTON (Reuters) - A quest by Republicans to open Alaska’s Arctic National Wildlife Reserve was slowed after a nonpartisan Senate official ruled late on Wednesday that the exploration was subject to environmental assessments by the Interior Department. Senator Lisa Murkowski, a Republican from Alaska and the head of the Senate energy panel, has been pushing a measure in the U.S. tax bill that would open a portion of the refuge on the coastal plain to two lease sales in 10 years for drilling. But the nonpartisan senate official took issue with the energy committee measure as it did not fully consider requirements under a national environmental law. The official ruled that oil exploration in the refuge is not exempt to an environmental law requiring the Interior Department to commission an assessment, a Democratic aide said. Such environmental assessments can take months or years to complete.   “This is good news for us because it could slow down or prevent drilling,” the Democratic aide said.   Republicans offered new language to the bill after the move and said that the drilling would still advance if the tax bill passes.  “There was a little hiccup, but they fixed it in the amendment they just filed tonight, so full steam ahead,” a Republican Senate aide said.    Murkowski said she was not concerned about procedural questions and that the issues would be fully resolved.   The Arctic reserve, protected by the federal government since 1960, is home to wildlife populations including caribou, polar bears and millions of birds that migrate to six of the seven continents. The U.S. Geological Survey estimates the area Republicans want to drill in has up to 12 billion barrels of recoverable crude. 