"A partial freeze of enrichment, as we're hearing, is not a freeze. As called for in UN Security Council resolutions, all of Iran's enrichment - the key bomb-making technology - should be ceased," he said.
Royce said the US run the risk of seriously weakening the sanctions structure painstakingly built-up against Iran over years.
"Once weakened, it will be harder to ratchet up the economic pressure on Iran than it will be for the Iranians to ratchet up their nuclear program," he said.
