Sina Corp, China's biggest Internet portal, forecast a record quarterly sales drop, citing a "challenging" online advertising market.

The company projected yesterday third-quarter sales will drop as much as 14 percent to $91 million, missing the $96 million median of nine analyst estimates compiled by Bloomberg.

The sales outlook is below what Shanghai-based Sina had expected, Chief Executive Officer Charles Chao said in a conference call, without specifying the previous forecast.