The United States intends to act next month to have the United Nations impose penalties on Iran for refusing to suspend its enrichment of uranium, a State Department official said Thursday.

"They will be well-deserved," Undersecretary of State Nicholas Burns told reporters. "It's not a mystery to the Iranians what is going to happen."

U.S. officials did not specify the proposed punishment.

Beyond the nuclear program, Iran supports Hezbollah as well as other terrorist organizations and has played a destabilizing role in the Middle East, said a department spokesman, Tom Casey.

The Security Council has said Iran faces penalties if it does not suspend uranium enrichment, an important step in making nuclear weapons.

Iran has until the end of the month for an official response. Tehran also had said it would reply by Tuesday to a proposal by the United States and the European Union for concessions that include Washington's supplying of some civilian nuclear energy.

Some critics urged the Bush administration to get on with negotiations with Iran.

A group of 22 former military officials and retired diplomats said President Bush immediately should open discussions. Thursday's letter also cautioned against any consideration of the use of military force.

"An attack on Iran would have disastrous consequences for security in the region and U.S. forces in Iraq, and it would inflame hatred and violence in the Middle East and among Muslims everywhere," the letter said.

Iran contends its enrichment and other nuclear programs are civilian in nature.

"We certainly want to give the Iranians the chance to take this last opportunity to accept the offer that is on the table," Casey said.

Burns said the U.S. wants to move quickly in September on the proposed U.N. penalties. He said the role of Iran in the Middle East has raised concerns among Arab and other countries about Tehran's 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."

As for the cease-fire in Lebanon between Israel and Hezbollah, Burns said Iran and Syria, the principal backers of the Hezbollah militia, "have a responsibility to respect the peace."