VIENNA, Austria, AP -A package of incentives presented Tuesday to Iran includes a provision for the United States to supply Tehran with some nuclear technology if it stops enriching uranium — a major concession by Washington, diplomats said.

The offer was part of a series of potential rewards offered to Tehran by European Union foreign policy chief Javier Solana, according to the diplomats, who were familiar with the proposals and spoke to The Associated Press on condition of anonymity because they were disclosing confidential details of the offer.

The package was agreed on last week by the United States, Britain, France, China and Russia — the five veto-wielding members of the U.N. Security Council, plus Germany, in a bid to resolve the nuclear standoff with Iran.

The incentives include European offers of help in building nuclear reactors for a peaceful energy program, the diplomats said.

That European offer of light-water reactors meant for civilian nuclear energy purposes was revealed last month, but there had been no suggestion that the Americans would also agree to help build Iran’s civilian nuclear program if Tehran freezes enrichment and agrees to negotiations.

Diplomats had revealed Monday that the United States had upped the ante on what had initially been a European offer by agreeing to provide Boeing aircraft parts for Iran’s aging civilian fleet. Just days before, Washington broke with decades of official policy of no high-level diplomatic contacts with Tehran, announcing it was ready to join in multinational talks with the Islamic republic over its nuclear program.

But one of the diplomats described the U.S. nuclear offer as particularly significant because it would in effect loosen what has been a decades-long American embargo on giving Iran access “dual use” technologies — equipment and know-how that has both civilian and military applications.

Both diplomats said Solana withheld the other part of the package — a series of measures meant to penalize Tehran if it does not relinquish enrichment, which can generate power but also can be used to make weapons-grade uranium for the core of nuclear warheads.

Solana “carried a message” about potential penalties but withheld the specific threats — including the possibility of U.N. sanctions — so as not to jeopardize the “positive” atmosphere, one of the diplomats said.

Those possible penalties include selective U.N. Security Council sanctions such as travel bans of Iranian government figures and a freeze of their foreign assets. But in a bow to Russia and China, they contain no threat of military action, diplomats have said.