Secretary of State Condoleezza Rice said Thursday she would not try to dictate how U.S. allies enforce sanctions on North Korea for its nuclear program, and there were signs South Korea wouldn't quickly embrace Washington's approach. "The key is to live up to the obligation that all of us undertook" to bar North Korea from exporting nuclear technology or receiving overseas help for its nuclear program, Rice said after meetings with South Korea's president and top diplomat.South Korea and China are the communist North's closest neighbors and trading partners, accounting for two-thirds of its foreign commerce. Both nations are pledged to carry out U.N. restrictions approved after North Korea's Oct. 9 test explosion of a small nuclear device, but they have hedged on details. Rice visits Chinese leaders Friday in Beijing.