WASHINGTON — President Barack Obama’s reluctance to give military aid to Syrian rebels may be explained, in part, in three words: Iranian nuclear weapons.

For the first time in years, the United States has seen a glimmer of hope in persuading Iran to curb its nuclear enrichment program so it cannot quickly or easily make an atomic bomb. Negotiations resume this week in Almaty, Kazakhstan, where encouraging talks in February between six world powers and the Islamic Republic ended in what Iranian diplomat Saeed Jalili called a “turning point” after multiple thwarted steps toward a breakthrough.

But Tehran is unlikely to bend to Washington’s will on its nuclear program if it is fighting American-supplied rebels at the same time in Syria. Tehran is Syrian President Bashar Assad’s chief backer in the two-year civil war that, by U.N. estimates, has left at least 70,000 people dead. Iranian forces are believed to be fighting alongside the regime’s army in Syria, and a senior commander of Iran’s elite Revolutionary Guard force was killed outside Damascus in February.

Russia also is supplying Assad’s forces with arms. And the U.S. does not want to risk alienating Russia, one of the six negotiating nations also seeking to limit Iran’s nuclear program.