WASHINGTON — Dismissing economic sanctions that are beginning to bite, Iran's supreme leader said Friday that his country wouldn't bow to Western demands that it stop enriching uranium and warned that a war over its nuclear program would be "10 times more harmful" to the United States.

"The Americans and others should, and do, know that we have our own threats to confront the military threats and oil sanctions and when necessary. We will make use of them at the right time," declared Ayatollah Ali Khamenei, according to an English transcript of his remarks posted on his official website.

Khamenei delivered the fiery, nationally broadcast sermon at Friday prayers at Tehran University amid rising tensions over Iran's nuclear program, which Tehran contends is producing uranium fuel for civilian nuclear reactors and the United States and other powers charge is secretly developing a nuclear warhead.

The Obama administration is growing increasingly concerned that Israel — dubious that sanctions will force Iran to halt its program before it attains the ability to build a bomb — will strike Iranian nuclear facilities in the coming months. The White House also has repeatedly refused to rule out U.S. military action as a last resort.

While Khamenei's speech may have been designed to warn off the United States and Israel, his harsh tone may primarily have been aimed at rallying domestic support for his unpopular regime, especially as Iran marks the anniversary of the 1979 Islamic revolution. The country also is heading into parliamentary elections next month with the sanctions stoking inflation, choking off hard currency supplies and forcing a devaluation of the Iranian rial.

Moreover, nothing that Khamenei said dampened expectations among U.S. and European officials and experts that Tehran may soon accept an invitation to renew international negotiations on its nuclear program — something that Iranian officials insist they favor.

"Nothing he (Khamenei) said is terribly surprising or different than what he's said in the past," said Robin Wright of the U.S. Institute of Peace. "I expect the Iranians to go back to the table."

In his rare public sermon, Khamenei declared that Iran won't heed U.N. demands to suspend uranium enrichment despite four rounds of U.N. sanctions and separate measures by the United States and the European Union designed to choke off Iranian oil sales, the country's main revenue source.

"The goal of these sanctions is to force Iran to back down, but Iran will not back down," Khamenei declared.

U.S. officials "constantly make threats, saying all options are on the table, that is to say even a military attack. This military threat is harmful to America and an actual war would be 10 times more harmful to America," he said.

Khamenei didn't elaborate on how Iran might respond. But Iran has threatened to blockade the Strait of Hormuz — the narrow waterway at the southern end of the Persian Gulf through which 40 percent of the world's seaborne oil travels — a development that could send world oil prices skyrocketing, dealing a major blow to the U.S. economy just as it's recovering from the Great Recession.

Iran also could launch ballistic missile attacks against U.S. forces in the region and Israel, and sponsor terrorist strikes against U.S. and European targets around the world.