The United States will press for new, tougher United Nations sanctions against Iran when diplomats hold a strategy session next week with five other nations that have sometimes reluctantly used the world body's punitive powers against oil-rich Iran. The State Department said the six nations will meet on September 21 in Washington. Undersecretary of State Nicholas Burns, the Bush administration's lead negotiator on Iran sanctions, will represent the United States. The UN's five permanent members, all nuclear powers, plus Germany have tried for nearly two years to use sanctions or the threat of them to persuade Iran to drop disputed nuclear work. Two rounds of mild sanctions, meant more to shame Iran than to pinch it economically, have not slowed or stopped the disputed activities. The United States, which has been estranged from Iran since the 1979 storming of the US Embassy in Teheran, has been the prime mover for world sanctions. US diplomats have not been specific about what they want the UN Security Council to try next. The six powers may consider a new round of sanctions in October, and some European diplomats are predicting contentious negotiations.