Iran was last known to have enriched uranium up to 60 percent purity, but a recent Bloomberg News report that International Atomic Energy Agency (IAEA) inspectors found uranium enriched to 84 percent, while strongly denied by Tehran, has sparked consternation.

Uranium enriched to around 90 percent purity is considered nuclear weapons-grade.

Iran has "advanced very far to the point where it would only be a matter of weeks before they can enrich to 90 percent, if they chose to cross that line," Burns told CBS's "Face the Nation," calling the progress "quite troubling."

He added, however, that the United States did not believe that Iran's supreme leader, the Ayatollah Ali Khamenei, had decided to "resume the weaponization program that we judge that they suspended or stopped at the end of 2003."

Tehran has repeatedly insisted that it is not planning to build a nuclear bomb.

A multi-nation 2015 accord promised Iran sanctions relief in exchange for cutting back its nuclear program.

But Iran started stepping up its nuclear activities in 2019, a year after the United States under then-president Donald Trump pulled out of the landmark deal and reinstated sanctions. Negotiations to revive the accord have stalled.

Burns said Iran was "still a ways off... in terms of their ability to actually develop a weapon" but said advancements in enrichment and missile systems that would be able deliver a nuclear weapon are "growing at a worrisome pace."

Another point of concern is that Russia is proposing to help Iran's missile program, Burns noted, reiterating as well the US belief that Moscow is also considering sending fighter jets to Iran.

Russia and Iran have expanded their military cooperation, with Tehran shipping growing quantities of weaponry to Moscow for use in the invasion of Ukraine.

The two states' cooperation is "moving at a pretty fast clip in a very dangerous direction," Burns said.

"That creates obvious risks not only for the people of Ukraine -- and we've seen the evidence of that already -- but also risks to our friends and partners across the Middle East as well."

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