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, by entering what would amount to a proxy war in Syria.

The White House has at least for now put the nuclear negotiations ahead of intervening in Syria, according to diplomats, former Obama administration officials and experts. Opposition forces in Syria are in disarray and commanded in some areas by a jihadist group linked to al-Qaida. Preventing Iran from building a nuclear bomb remains a top priority for the Obama administration, which has been bent on ending wars — not opening new military fronts.

“I think that the United States has not taken a more active role in Syria from the beginning because they didn’t want to disturb the possibility, to give them space, to negotiate with Iran,” Javier Solana, the former European Union foreign policy chief, said Monday at a Brookings Institution discussion about this week’s talks. Solana, who was a top negotiator with Tehran in the nuclear program until 2009, added, “They probably knew that getting very engaged against Assad, engaged even militarily, could contribute to a break in the potential negotiations with Tehran.”

Solana also warned of frostier relations between Moscow and Washington that could scuttle success in both areas. “With Russia, we need to be much more engaged in order to resolve the Syrian problem and, at the end, the question of Tehran,” he said.

Adding to the mix is the unpredictable relationship between the U.S. and China, which has been leery of harsh Western sanctions on Iran and is expected to follow Russia’s lead on the nuclear negotiations. Without Russia and China’s support, experts say, the West will have little success in reaching a compromise with Iran.

“Resolving the nuclear impasse with Iran is the biggest challenge this year in the Middle East, and that requires careful handling of not only Iran, but Russia and China,” said retired Ambassador James F. Jeffrey, who followed the negotiations closely as the top U.S. envoy to Baghdad last year. “Decisions on Syria and other international questions certainly will be taken in this context.”

The negotiations have indirect, if wide-reaching, links to regional affairs that include Syria but also go beyond, including the U.S. military presence in the Persian Gulf, Washington’s uneasy detente with Baghdad and Israel’s undeclared nuclear arsenal — the only one of its kind in the Mideast.