WASHINGTON — Iran has barred U.N. inspectors from a key underground nuclear installation, breaching its obligations under the nuclear nonproliferation treaty.

The news emerged on the eve of Iran’s formal response today to incentives to end its nuclear program offered by the five permanent members of the U.N. Security Council: Britain, France, America, Russia, and China.

As international pressure on Tehran mounted, President Bush warned that the Iranians would pay a significant diplomatic price if they failed to agree.

“There must be consequences if people thumb their nose at the United Nations Security Council, and we will work with people in the Security Council to achieve that objective,” he said.

Western diplomats have made clear that an Iranian rejection will result in a demand for U.N. sanctions to halt Iran’s 20-year-old nuclear program. The council has set an August 31 deadline for a response.

U.N. officials described as unprecedented the decision to block their inspection of the uranium enrichment plant in Natanz. Iran is obliged under the 1968 nonproliferation treaty to inform the U.N. of progress at its nuclear sites.

Iran’s supreme leader, Ayatollah Ali Khamenei, said yesterday that the nuclear program would continue and accused “arrogant powers and the U.S.” of putting pressure on his country, even though they knew no weapons program exists.

The incentives would allow Iran to buy American aerospace and agricultural technology while Russia would supply uranium for civilian reactors.