US President Barack Obama and Chinese leader Xi Jinping have ended a two-day summit described by a US official as "unique, positive and constructive". US National Security Advisor Tom Donilon said Mr Obama had warned Mr Xi that cyber-crime could be an "inhibitor" in US-China relations. He also said that both countries had agreed that North Korea had to denuclearise. The talks in California also touched on economic and environmental issues. The two leaders spent nearly six hours together on Friday and another three hours on Saturday morning at the sprawling Sunnylands retreat in California.