UPDATE 1-Vietnam, China embroiled in South China Sea standoff

Vietnamese and Chinese ships have been embroiled in a weeks-long standoff near an offshore oil block in disputed waters of the South China Sea, which fall within Vietnam's exclusive economic zone, two Washington-based think-tanks said on Wednesday.China's U-shaped "nine-dash line" marks a vast expanse of the South China Sea that it claims, including large swathes of Vietnam's continental shelf where it has awarded oil concessions.