ASPEN, Colo. — Iran is talking more about getting a nuclear bomb and has made strides in developing a key aspect of a weapon since about April, when Israel and its allies overpowered a barrage of Iranian airstrikes targeting Israel, two top Biden administration officials said Friday.

White House national security adviser Jake Sullivan speaks during the daily briefing at the White House in Washington on May 22.  Susan Walsh/Associated Press 

Secretary of State Antony Blinken and national security adviser Jake Sullivan, speaking separately at a security forum in Colorado, said the U.S. was watching closely for any signs that Iran had made a decision to pursue actual weaponization of its nuclear program.

However, Sullivan said, “I have not seen a decision by Iran to move” in a way that signals it has decided to actually develop a nuclear bomb right now.

“If they start moving down that road, they’ll find a real problem with the United States,” Sullivan told reporters at the Aspen Security Forum, which draws U.S. policymakers, journalists and others.

Iran resumed progress on its nuclear program after the Trump administration ended U.S. cooperation with a 2015 deal that gave Iran relief from sanctions in return for allowing tougher oversight of the program.

Iran says its nuclear program is for civilian purposes. The U.S. and others in the international community believe Iranian Supreme Leader Ali Khamenei long has held off from giving any final go-ahead for Iranian scientists to develop a nuclear weapon.

But Iran’s poor performance with conventional weapons on April 13, when it launched its first direct attack on Israel as part of a dayslong exchange of strikes, has had observers watching for any increased Iranian interest in pushing forward with a nuclear weapon.

At the time, Israel said it, the United States and other allies shot down 99% of some 300 missiles and drones launched by Iran in response to an alleged Israeli strike that killed two Iranian generals.

“What we’ve seen in the past weeks and months is Iran is actually moving forward” on developing fissile material, Blinken said Friday during a panel talk. Fissile material could be used to fuel a bomb.

Blinken blamed the decision by the Trump administration to pull out of the nuclear agreement. Trump called the deal “defective at its core” when he ended U.S. participation in 2019.

“Instead of being at least a year away from having breakout capacity to produce fissile material for a nuclear weapon, they’re probably one or two weeks away from doing that,” Blinken said, adding that “where we are now is not a good place.”

“Now, they haven’t produced a weapon itself, but … you put those things together – fissile material, an explosive device – and you have a nuclear weapon,” he said.

The United States is watching “very, very carefully” for any sign that Iran was working on the weapon side of producing a bomb, he said. The U.S. is also working on the diplomatic side to contain any further effort, Blinken said.

Meanwhile, Sullivan said that the U.S. has noted “an uptick of public commentary from Iranian officials musing about that possibility” since the April attacks and counterattacks between Iran and Israel and its allies.

“That was new. That was something that got our attention,” he said.