LONDON - Oil markets are beginning to recover but the scale of global oversupply means prices may rise only slowly, the chief executive of Royal Dutch Shell Plc said on Tuesday. “I see the first mixed signs for recovery of oil prices,” Ben van Beurden told an oil industry conference in London. “But with U.S. shale oil being more resilient than we originally thought and a lot of oil still in stock, it will take some more time to rebalance demand and supply,” he added. Oil prices have collapsed over the last year in the face of heavy oversupply, with benchmark Brent crude falling to below $50 a barrel from a high above $115 in June 2014. The Organization of the Petroleum Exporting Countries led by Saudi Arabia has increased production in an attempt to build market share, leaving some other producers, including shale companies in North America, operating below break-even costs. Van Beurden said many U.S. oil producers would struggle to refinance while prices remained so low, leading to lower output in the future: “Producers are now looking for new cash to survive and they will probably struggle to get it.” Longer-term, there was a risk that low levels of global production could bring a spike in oil prices, he said. If prices remained low for a long time and oil production outside OPEC and the United States declined due to cuts to capital expenditure, there was not likely to be any significant spare capacity left in the system, he said. “This could cause prices to spike upwards, starting a new cycle of strong production growth in U.S. shale oil and subsequent volatility,” van Beurden said.