Oil prices declined Thursday on concerns over China's economic slowdown and potential supply increase from Libya. The purchasing managers' index (PMI) for the manufacturing sector rose to 50.4 last month, up from 50.3 in March, according to a statement jointly released by the National Bureau of Statistics (NBS) and the China Federation of Logistics and Purchasing. Although China's manufacturing growth continued to rise in April, the sub-index for export new orders dropped, fueling market worries over China's economic slowdown in the first quarter. Moreover, an oil port in Libyan, which has been closed for about 10 months due to protests, is said to have started loading oil again from Thursday. In the previous session, oil prices dropped after the Energy Information Administration (EIA) on Wednesday reported U.S. crude inventories rose 1.7 million barrels to 399.4 million barrels for the week ending April 25, the highest level in 83 years. The markets were also dragged by the below-expectation U.S. gross domestic product (GDP) in the first quarter. The real GDP increased at an annual rate of 0.1 percent in the first quarter, the slowest pace since the fourth quarter of 2012, according to the advance estimate released by the Commerce Department Wednesday. Light, sweet crude for June delivery fell 0.32 dollar to settle at 99.42 U.S. dollars a barrel on the New York Mercantile Exchange, while Brent crude for June delivery lost 0.31 dollar to close at 107.76 dollars a barrel.