Washington's recent bid to persuade North Korea to return to nuclear disarmament negotiations included a personal letter to leader Kim Jong Il written by President Barack Obama, a senior US official said.Envoy Stephen Bosworth took the letter to Pyongyang last week, handing it over to North Korean officials during the Obama administration's first bilateral talks since the S president took office, the official said in Washington.The official, who spoke Tuesday on condition of anonymity because of the sensitive nature of the diplomacy, would not describe the contents of the lette but said they fit with Bosworth's general message."The North Koreans have a choice: continued and further isolation or benefits for returning to the six-party talks and dismantling their nuclear weapons program," the official said. The official was not aware whether Kim had responded.Bosworth did not meet Kim during his three-day trip. But after leaving Pyongyang, he said he conveyed Obama's message that the United States is ready to work with its allies to offer North Korea "a different future" if Pyongyang chooses to rejoin the disarmament talks and take irreversible steps toward dismantling its atomic program.Both Washington and Pyongyang agreed on the need to resume the stalled talks but the North did not make a firm commitment on when it would rejoin the negotiations.Amid a flurry of disarmament diplomacy, Chinese Vice President Xi Jinping was scheduled to visit South Korea later Wednesday for talks with President Lee Myung-bak and other South Korean officials. Xi and Lee are expected to discuss North Korea on Thursday.Separately, South Korea's chief nuclear envoy, Wi Sung-lac, plans to leave for Moscow later Wednesday for consultations with Russian officials on North Korea's nuclear program, Foreign Minister Yu Myung-hwan told reporters on Wednesday.The flurry of trips comes amid a report that Kim is suffering from chronic laryngitis - probably because of excessive smoking and drinking - and he can't work without resting every other day.The illness worsened last month, though the 67-year-old leader has recovered much from last year's reported stroke and a kidney disease, said the Seoul-based Open Radio for North Korea, a radio station specializing in North Korean news.The station cited an unidentified "high-level source" in the North. South Korean officials could not confirm the report.Kim's health has security implications beyond the Korean peninsula because of fears of a possible power struggle if he were to die without naming a successor, a scenario that could affect the North's nuclear programs and the disarmament process.The disarmament process had yielded pacts promising North Korea much-needed aid and other concessions in return for step-by-step disarmament. However, Pyongyang walked away from the talks this year in anger over U.N. Security Council condemnation of a rocket launch widely seen as a test of its long-range missile technology.The UN Security Council tightened sanctions in June after North Korea conducted an underground nuclear test, its second, in defiance of an earlier ban. The sanctions, aimed at derailing North Korea's nuclear weapons program, ban the country from developing its nuclear program and selling conventional arms.Meanwhile, the North told Bosworth that U.N. sanctions imposed on the communist regime for its nuclear defiance must be lifted, Yonhap reported Wednesday.Yu, the foreign minister, reaffirmed that the sanctions would remain in place while making diplomatic efforts to revive the stalled talks, noting the US and its regional powers would not reward Pyongyang just for returning to the negotiating table.The North's reported demand comes as Thai authorities are inspecting 35 tons of weapons seized from a cargo plane loaded in North Korea - the latest known case of Pyongyang's illicit weapons trade in violation of UN sanctions.Thai officials impounded the Ilyushin IL-76 transport plane Saturday after authorities discovered explosives, rocket-propelled grenades and components for surface-to-air missiles.North Korea is believed to earn hundreds of millions of dollars every year by selling missiles, missile parts and other weapons to countries such as Iran, Syria and Myanmar.