China will likely soon overtake Japan as the U.S.'s third-biggest export market, The Wall Street Journal reported, citing a top U.S. Commerce Department official as he began a visit to press for wider access to China's market for U.S. banks and other companies.   Franklin Lavin, U.S. undersecretary of commerce for international trade, US is seeking greater clarity from China on its commitments to allow foreign banks to start taking local-currency deposits from individuals by December.   Lavin would also push for better treatment for Citigroup Inc. The opening of local-currency business to foreigners, a step promised in China's 2001 World Trade Organization accession agreement, is supposed to remove the last big barrier for foreign banks in China. But Lavin echoed concerns expressed by foreign bankers here that Beijing could erect new impediments, such as requiring foreign banks to incorporate their Chinese branch networks inside the country -- a potentially costly and complicated procedure, said the paper. US exports to China rose almost 37 percent year-on-year in the first five months of 2006.