Obama calls for unity to tackle nuclear terrorism A 53 nation nuclear security summit wrapped up in South Korea on Tuesday with a warning by US President Barack Obama that the threat of nuclear terrorism "remains." The two biggest nuclear headaches, Iran and North Korea, were not on the summit's agenda and neither country was invited. But leaders did make a vague call for vulnerable atomic material to be secured within four years. US President Barack Obama called for closer co-operation to tackle the threat: "There are still too many bad actors in search of this dangerous material and these materials are still vulnerable in too many places. It would not take much - just a handful or so of these materials - to kill hundreds of thousands of innocent people. And that's not an exaggeration. That's the reality that we face," he added. A communique issued at the end of the summit reaffirmed a commitment to minimise stockpiles of highly enriched uranium and plutonium while preventing the trafficking of radioactive material. More about: Barack Obama, Nuclear Energy, Seoul, South Korea, Summit