The United States intends to move very quickly in early September to impose UN sanctions on Iran for refusing to suspend its enrichment of uranium, a senior State Department official said Thursday."They will be well-deserved," Under-Secretary of State Nicholas Burns told reporters. "It's not a mystery to the Iranians what is going to happen."The UN Security Council already has said Iran faced sanctions if it did not suspend uranium enrichment, a key step in making nuclear weapons.Iran has until the end of the month to respond officially. It also had said it would reply by next Tuesday to a proposal by the United States and the European Union for concessions that include US supply of some civilian nuclear energy.Iran rejects allegations that it is trying to develop nuclear weapons. It says its enrichment and other programs are civilian in nature."We want to move very quickly in the first part of September," Burns said, referring to threatened UN sanctions.He said the role of Iran in the Middle East has raised concerns among Arab and other countries about its intentions."There is broadened concern about the policy of a country that flexes its muscles," he said. "Iran wants to be the dominant country in the region."Burns, who played a leading role in US diplomacy over the conflict in Lebanon between Israel and Hizbullah, said, "I talk to Arab governments, and there is increased concern about Iran."Iran and Syria, the principal backers of the Hizbullah militia, "have a responsibility to respect the peace," Burns said, referring to the cease-fire declared last week by the UN Security Council.He said the international force the United Nations is arranging to send to southern Lebanon will be strong and heavily armed. While it will not disarm Hizbullah - Burns said that is a task for the Lebanese government - it will keep Hizbullah from rebuilding its fortifications across the border from Israel.