The United States’ main concern with North Korea is its nuclear program, the US Defense Secretary Ash Carter said on Friday, adding that American officials are eyeing the Korean peninsula daily."The biggest worry about North Korea has been - other than its sort of strange and isolated behavior - has been its nuclear program," Carter, speaking from Davos, Switzerland, told CNBC in an interview. Carter said US officials are keeping a close eye on the demilitarized zone on the Korean peninsula. Although prepared to fight if needed, the United States is focused on avoiding a military conflict there, he said."It's something we have to watch every single day," he said.