<span style="color: #222222;">The United States on Friday warned its citizens that they could face arbitrary action by authorities when they visit China. </span><br />'' <br />'' <span style="color: #222222;">Updating its travel advisory, the State Department maintained its previous guidance that Americans should exercise increased caution in China. </span><br />'' <br />'' <span style="color: #222222;">It also advised caution over last-minute security checks and curfews in Xinjiang and Tibet. </span><br />'' <br />'' <span style="color: #222222;">The warning comes after China last month detained two Canadians on charges of activities that endanger China's security. </span><br />''