The Pentagon chief also said the United States is "watching" Iran's enriched uranium, which Washington targeted in strikes last year, and will take action if Tehran does not give it up.
The US war against Iran has "completely" destroyed the country's ability to build missiles and other sophisticated weaponry, Defence Secretary Pete Hegseth said on Wednesday.
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The United States and Iran agreed to a two-week ceasefire the previous evening, pausing a nearly six-week US-Israeli military campaign against the Islamic republic.
"We finished completely destroying Iran's defence industrial base, a core pillar of our mission," Hegseth told reporters.
"They can no longer build missiles, build rockets, build launchers or build (drones), their factories have been razed to the ground, set back in historic fashion," he said.
"Had Iran refused our terms, the next targets would have been their power plants, their bridges and oil and energy infrastructure," Hegseth said, after President Donald Trump had threatened to wipe out civilisation in the country if it did not agree to a deal.
General Dan Caine, the top US military officer who spoke alongside Hegseth, provided details on the destruction of Iran's defence industry, which were not as total as Hegseth described.
Caine said around 90% of Iran's weapons factories, more than 80% of its missile facilities and nearly 80% of its nuclear industrial base had been hit.
"Every factory that produced Shahed one-way attack drones was struck," he said, referring to weapons Iran has repeatedly employed during the course of the conflict and which it has also provided to Russia for use in Ukraine.
US forces hit more than 13,000 targets during the war, destroying 80% of Iran's air defences as well as "more than 450 ballistic missile storage facilities" and "800 one-way attack drone storage facilities," said Caine.
And while a temporary ceasefire has been reached, the US military is prepared if it collapses, he added.
"Let us be clear, a ceasefire is a pause and the joint force remains ready, if ordered or called upon," Caine said.
Hegseth warned that "we stand ready in the background to ensure that Iran upholds" the terms.
"We're going to make sure Iran complies with this ceasefire and then ultimately comes to the table and makes a deal," he said.
The Pentagon chief also said the United States is "watching" Iran's enriched uranium, which Washington targeted in strikes last year, and will take action if Tehran does not give it up.
"We know exactly what they have, and they know that, and they will either give it to us" or "we'll get it, we'll take it, we'll take it out," he said, raising the possibility of further US strikes.
Earlier on Wednesday, US President Donald Trump said Washington would "work closely with Iran," which "has gone through what will be a very productive regime change," as the fragile two-week truce came into effect.
In a post on Truth Social, the US president said "there will be no enrichment of uranium, and the United States will, working with Iran, dig up and remove all of the deeply buried (B-2 bombers) nuclear 'dust'," likely referring to Tehran's uranium stockpiles.
"It is now, and has been, under very exacting satellite surveillance (Space Force). Nothing has been touched from the date of attack."
"We are, and will be, talking tariff and sanctions relief with Iran. Many of the 15 points have already been agreed to. Thank you for your attention to this matter," Trump concluded.
The United States and Iran agreed to a two-week ceasefire late on Tuesday night, barely an hour before Trump's deadline for Iran to capitulate was set to expire.
Trump pulled back on earlier apocalyptic threats that a "whole civilisation will die” if Tehran failed to reach a deal, holding off on his ultimatum that was to result in attacks on Iranian bridges and power plants.
Negotiations in Pakistan
Iran's Supreme National Security Council said it has accepted the ceasefire and would negotiate with the US in Islamabad beginning Friday.
Iranian Foreign Minister Abbas Araghchi said passage through the Strait of Hormuz would be allowed under Iranian military management. It was not immediately clear whether that meant Iran would completely loosen its chokehold on the waterway.
Neither Iran nor the United States said when the ceasefire would begin, and attacks took place in Israel, Iran and across the Gulf region early Wednesday.
However, US Vice President JD Vance on Wednesday welcomed a "fragile truce" with Iran, urging Tehran to negotiate in "good faith" to reach a long-term deal and warning Trump was "not one to mess around".
"If the Iranians are willing in good faith to work with us, I think we can make an agreement," Vance said during a visit to Budapest.
He added, if the Iranians were not to come to the negotiating table, "they're going to find out that the president of the United States is not one to mess around. He's impatient. He's impatient to make progress."
In the version of its 10-point ceasefire plan released in Persian, Iran included the phrase “acceptance of enrichment” regarding uranium needed for its nuclear programme, but for reasons that remain unclear, that phrase was missing from English versions shared by Iranian diplomats with journalists.
Trump initially had said Iran proposed a “workable” 10-point plan to end the war, but he later called it fraudulent, without elaborating. Trump has said ending Iran’s nuclear programme entirely was a key point of the war.