Hong Kong leader warns of national security threats after US citizen sentenced by China.

Hong Kong's Chief Executive John Lee has warned the city to "stay vigilant" to security risks after China sentenced a 78-year-old US citizen and Hong Kong permanent resident to life in prison on spying charges. The Hong Kong government and the court in Suzhou that tried John Shing-Wan Leung have not released details of the charges. Leung's sentencing threatens to further exacerbate already strained ties between Beijing and Washington.