London, Feb 23 -- At least 21 Chinese grounded boats have been rescued on the Mekong River, a cross-border river flowing out of China from south-west China's Yunnan Province, the marine affairs bureau in Yunnan said today. The bureau's subsidiary in Xishuangbanna Dai Autonomous Prefecture, which borders Laos and Myanmar, has stopped issuing pass permits to vessels and repeatedly sent warnings to foreign vessels, telling them of the danger. "The river is at its lowest level in 50 years. The boats grounded in the river sections in Laos and Myanmar have been pulled to relatively safer waters on the river or to docks in Thailand. But the vessels are not completely out of danger," said Liu Rongming, deputy chief of the Xishuangbanna bureau. He said Chinese rescuers have sent relief goods and money to the crews. The river's flow is only half the normal level, as a rare, large-scale drought - the worst in 50 years - hits south-west China, according to the bureau. The river, called Lancang Riv