The United States is considering limiting hi-tech exports to Russia's Arctic oil and gas industry as part of plans to strengthen sanctions against Moscow over the crisis in Ukraine, a U.S. envoy said on Wednesday. On the eve of a NATO summit in Wales, the United States is expected to join the European Union in imposing new sanctions on Russia after EU leaders agreed at the weekend that the direct engagement of Russian troops in Ukraine in support of separatist rebels merited such a step. "We are looking at further tightening of restrictions on energy exports and the licensing of hi-tech exports, not only to the energy sector but specifically to Arctic oil and gas," the U.S. ambassador to the European Union, Anthony Gardner, told Reuters in Brussels.