BEIJING—China's ruling communists are sticking to their trademark secrecy by keeping mum on the dates of a crucial leadership conference due this fall.

Party officials at a news conference Tuesday insisted they were committed to greater transparency.

But when questioned about when the 18th national party congress would start, the deputy head of the party's Organization Department, Wang Jingqing, merely said it was designated for the second half of the year.

Wang also said he did not know whether the all-powerful Politburo Standing Committee would remain at its current size of nine seats or shrink to seven.

The congress marks a once-in-a-decade leadership turnover, with seven of the Standing Committee members set to step down, and a new crop of technocrats brought in to steer the world's second largest economy.