US Democratic presidential candidate Barack Obama has said Iran is a "great threat" and has called for tighter sanctions after it test-fired nine missiles.Mr Obama said on a US TV show: "Iran is a great threat. We have to make sure we are working with our allies to apply tightened pressure on Iran."Iran said it fired the missiles as a warning to the US and Israel that it was ready to retaliate if they attacked over its disputed nuclear projects.Mr Obama has said that if he were president, he would combine more direct diplomacy with the threat of much tougher economic sanctions.He said: "I think what this underscores is the need for us to create a kind of policy that is putting the burden on Iran to change behavior, and frankly we just have not been able to do that over the last several years."The Ilinois senator cited reports that US exports to Iran have increased under President George W Bush even as the administration has toughened its rhetoric.News of Iran's missile test came as the G8 expressed "serious concern" over Iran's failure to comply with UN Security Council resolutions calling for Tehran to suspend all enrichment-related activities.Last month, six major world powers offered a package of economic incentives to Iran if it halted nuclear activities. Tehran's formal response to that offer has not been made public.The G8 statement said: "We call on Iran to respond to the updated incentives package in a constructive manner."The statement added: "We also urge Iran to act in a more responsible and constructive manner in the region, particularly in the context of the Middle East peace process and the stability of Iraq and Afghanistan."The White House said Iran's development of ballistic missiles was in violation of UN Security Council resolutions but that the US was committed to pursuing a diplomatic solution to Iran's nuclear ambitions.