India has sparred diplomatically with China in the past over its gas and oil exploration block off the coast of Vietnam. China claims virtually the entire mineral-rich South China Sea and has stepped up its military presence there. Part of block 128 is in the U-shaped "nine-dash line" which marks the vast area that China claims in the sea, a route for more than $5 trillion in trade each year in which the Philippines, Brunei, Malaysia and Taiwan also have claims.
