Japan was a key source of finance for the US during the Gulf War but it is cautious about supporting a new war.

Prime Minister Koizumi said: "The use of force is a last resort where there is no other options."

Japan is banned under its pacifist constitution from using force to settle international disputes.

It passed a controversial anti-terrorist law to enable it to provide logistical support to the US-led military operations in Afghanistan, but this would not cover similar involvement in Iraq.