The United States said Friday it won't let the recent progress in its nuclear diplomacy with North Korea affect its close relations with key ally South Korea. U.S. Secretary of State Hillary Rodham Clinton offered the reassurance after meeting South Korean Foreign Minister Kim Sung-hwan in Washington. Nuclear envoys from the North and South are both attending the non-official forum on security in Northeast Asia but have not held separate talks, although it appears Seoul was willing to have their envoy to meet his counterpart from Pyongyang. "I want to be very clear: Any effort by anyone to drive a wedge between the United States and the Republic of Korea will fail," Clinton told a news conference at the State Department, alongside Kim. We consult closely on all aspects of our diplomacy. Last week, North Korea agreed to a freeze of nuclear activities in return for U.S. food aid. But while the North has reached out to the U.S., it has called for a "sacred war" against the South, in response to recent military exercises by the allies. South Korean Unification Minister Yu Woo-ik called on North Korea Friday to hold talks with the South. The U.S. is also keen to see an improvement in inter-Korean relations. "I urge North Korea again to come forward for dialogue as soon as its internal situation stabilizes," Yu said at a forum in Seoul, referring to the power transition in North Korea from longtime ruler Kim Jong Il, who died in December, to his untested son Kim Jong Un. In New York, John Kerry, chair of the U.S. Senate Committee on Foreign Relations, said the transition in North Korea "perhaps makes it a moment of hope." Kerry, the most senior U.S. figure attending the New York conference, said there were opportunities for engagement with North Korea, but advised caution too. "We have been through this process before and it's proved disappointing on occasion, so I think you have to proceed cautiously," he told reporters on his way into the meeting.