The United States President Barack Obama has said North Korea must be "held to account" for the sinking of a South Korean warship. He said he stood with South Korean leader Lee Myung-Bak, and condemned Pyongyang's "irresponsible behaviour". His comments came shortly after the G8 group of industrialised nations meeting in Toronto condemned the sinking of the corvette Cheonan earlier this year. North Korea has denied attacking the ship. An international investigation concluded that it was sunk by a North Korean torpedo. <br/> <br/>The G8 leaders also criticised of North Korea – and Iran – over their nuclear activities, and they described the Israeli blockade of the Gaza Strip as "not sustainable". And they admitted that the global financial crisis had compromised efforts to meet UN targets for reducing world poverty. Canada is also hosting a summit of the wider G20 group of industrial and developing powers, which will will discuss the global economy and financial reform. <br/>