RIO DE JANEIRO -(Dow Jones)- Sentiment in the global iron ore market remained uncertain in April, even though prices for the steelmaking ingredient continued to firm over the first 20 days of the month, price provider The Steel Index said in its monthly bulletin, published late Friday. Spot reference prices for ore with 62% iron content, delivered to China, averaged $147.65 a dry metric ton for the month, up 2.1% on March, The Steel Index said. For most of the month, prices ranged between $145 and $150 a ton, although they started slipping at the end of the month amid fresh concerns over the European debt crisis and difficulties in predicting Chinese iron ore demand, it said. Iron ore inventories at Chinese ports started to decline in April as buyers turned to cheaper quayside material in preference to more expensive seaborne cargoes, the bulletin said. However, China's own ore production has been edging up since March--when it reached 100 million tons compared with imports of 62.9 million tons--as warmer weather has allowed operations to resume at mines in the north of China, The Steel Index said. This adds to the uncertainties over predicting Chinese iron ore import needs in coming months, following a 6% increase in imports from the year-earlier period in the first quarter, according to the price provider. During April, mining companies Vale SA (VALE, VALE5.BR), BHP Billiton Ltd., Rio Tinto PLC and Fortescue Metals Group all signed up to become members of China Beijing International Mining Exchange, a new electronic iron ore trading platform due to start trading May 8, The Steel Index said. The exchange will trade ores from Brazil, Australia, India, South Africa, Russia, Canada and other countries, according to the bulletin. -By Diana Kinch, Dow Jones Newswires, 55 21 7564 4495 diana.kinch@dowjones.com -0-