North Korea said it reached an unspecified agreement with the US this week on its nuclear weapons program, but America's top nuclear envoy said Friday it was not clear how these discussions would advance broader international arms talks. North Korea's Foreign Ministry said the talks between US envoy Christopher Hill and North Korea's main nuclear negotiator Kim Kye Gwan had been held "in a positive and sincere atmosphere and a certain agreement was reached there." No further details were given. But Hill, who arrived Friday in Seoul, said he wasn't aware what agreement the statement was referring to. He called the talks in Berlin "very useful discussions."