Prime Minister Manmohan Singh today expressed deep concern over the sharp rise in onion prices and said effective steps should be taken to bring down the prices to affordable levels. In a letter to the secretaries of Departments of Consumer Affairs and Agriculture, Dr. Singh has expressed deep concern over the extraordinary price rise.The Prime Minister wanted that steps should be taken expeditiously and the impact should be monitored on a day-to-day basis. Sources said, the PM's letter also made references to the wholesale and retail prices.