HONG KONG (Reuters) -A group of offshore creditors of China South City is set to file a lawsuit against the debt-laden developer's biggest state-owned shareholder for dues, four sources said, in what would be the first such case in the crisis-hit property sector. China South City earlier this month missed a principal payment of $11.25 million on a dollar bond due on Feb. 9, and is deemed by creditors to be in default on offshore debts worth $1.3 billion, two bondholder sources said. The group of creditors, who have formed a so-called ad-hoc group, is preparing to file the lawsuit in a Hong Kong court against state-owned Shenzhen SEZ Construction and Development Group Co., which owns 29% of the company, the four sources said.