Vice President JD Vance on Wednesday said that the US has seen evidence that Iran is trying to build a nuclear weapon.
His comments come a day before renewed talks between the US and Iran in Geneva over a deal on Tehran's nuclear programme.
"The principle is very simple: Iran cannot have a nuclear weapon," Mr Vance told reporters.
President Donald Trump used similar words during his State of the Union address to Congress on Tuesday night.
Mr Trump said Iran has already developed missiles that are a threat to European and American bases in the region and is working to build missiles that will soon reach the US.
“My preference is to solve this problem through diplomacy,” he said. “But one thing is certain, I will never allow the world's number-one sponsor of terror – which they are by far – to have a nuclear weapon.”
Meanwhile, the US issued new sanctions against more than 30 people and companies connected to Iran's so-called shadow fleet.
The fleet is a network of oil tankers and companies used by Iran to ship petroleum to foreign markets in defiance of international sanctions.
Scott Bessent, Secretary of the Treasury, said the Trump administration will continue to apply “maximum pressure” on Tehran. “Iran exploits financial systems to sell illicit oil, launder the proceeds, procure components for its nuclear and conventional weapons programmes, and support its terrorist proxies,” Mr Bessent said.
Eleven tankers, including the Panama-flagged Hoot and the Palau-flagged Alaa, had sanctions imposed, as did four people and about a dozen companies.
The State Department said the measures were against the “illicit funds that the regime uses to advance its malign and destabilising ends”.
The sanctions also hit networks that enable Iran's Islamic Revolutionary Guard Corps and Ministry of Defence to secure the materials and equipment needed for the production of ballistic missile and other weapons, the Treasury said.
