US President Barack Obama on Sunday demanded that China acknowledge North Korea's role in an attack on a South Korean naval vessel earlier this year.
An international inquiry found that North Korea torpedoed and sank the South Korean warship the Cheonan March 26. North Korea has denied it was behind the attack, and China has refused to allocate blame.
Obama said he understood Chinese President Hu Jintao's need to show restraint over his neighbour and ally, but "there's a difference between restraint and willful blindness to consistent problems".
"My hope is that President Hu will recognise as well that this is an example of Pyongyang going over the line in ways that just have to be spoken about," Obama told reporters at a Group of 20 nations summit in Toronto.
Obama said he was "very blunt" on the matter when he met with Hu Saturday on the sidelines of the summit. The US was "mindful that this is in China's backyard," but he warned that world governments would not be able to bring peace to the Korean Peninsula if China continued to ignore North Korea's actions.
"It is a bad habit that we need to break to try and shy away from ugly facts with respect to North Korea's behaviour ... under the illusion that that will somehow help to maintain the peace," he said.
South Korea has brought the issue before UN Security Council, which has struggled to reach agreement on a resolution because of China's reluctance to condemn its neighbour.
Obama said he would accept nothing less than a "crystal clear" acknowledgement from the Security Council of North Korea's attack.
"This is a situation where you have a belligerent nation that engaged in provocative acts against the other. I think it is very important that we are clear on that," Obama said.