The United States has called for tighter enforcement of UN sanctions against North Korea and for China to play a “responsible” role in easing the mounting tensions in the region. US Ambassador to the United Nations Susan Rice on Monday said that it will “confront the threat” posed by North Korea's new nuclear activities and the deadly attack on its southern neighbour last week.A UN Security Council sanctions committee meanwhile met to discuss efforts to implement actions already ordered against the North Korea after its nuclear tests in 2006 and 2009.