Oil has traded at $100 a barrel for the first time, with violence in Nigeria, Algeria and Pakistan, the weak US dollar and the threat of cold weather have all raised prices after the new year break. Light sweet crude rose $4.02 to $100 a barrel in New York, prompting a drop in shares and a surge in gold prices. There are concerns that the high price of oil will stoke inflation at a time when many central banks are trying to cut interest rates to stimulate growth. US shares had already been hit on Wednesday by figures showing that the manufacturing sector was contracting. While daily price rises have been blamed on unrest in oil-supplying countries such as Nigeria, an underlying and significant factor has been an increase in demand from China and India. “$100 is just the beginning,” said Zachary Oxman, senior trader at Wisdom Financial in California.