The Nikkei stock index extended gains early Tuesday afternoon, rising over 2 percent, supported by firm U.S. stock futures and buying spurred after the index's drop to a five-week low the previous day. As of 12:45 p.m., the 225-issue Nikkei Stock Average was up 536.05 points, or 2.08 percent, from Monday at 26,307.27. The broader Topix index was 37.40 points, or 2.06 percent, higher at 1,856.34. Every industry category on the top-tier Prime Market gained ground. Gainers were led by mining, oil and coal product, and air transportation issues.