A US Navy warship seized a "significant cache" of Iranian guided missile parts being sent to rebels in Yemen, US officials said on Thursday.
The seizure from a small boat, by the US Navy and US Coast Guard, happened last Wednesday in the northern Arabian Sea off the coast of Yemen.
The US Special Representative for Iran Brian Hook said it was the first time such sophisticated missile parts had been seized en route to the war in Yemen.
"The seizure includes sophisticated weapons, sophisticated components of anti-ship cruise missiles, land attack cruise missiles, air defence missiles, and anti-tank missiles," Mr Hook told a press briefing.
The Pentagon said earlier that a US warship had found “advanced missile components” on a stateless vessel and an initial investigation indicated the parts were of Iranian origin.
“A more thorough investigation is under way,” the Pentagon said.
"This discovery is yet more proof of Iran’s efforts to inflame conflicts in the region by proliferating deadly weapons to its proxies," Mr Hook said. "It is also further evidence of how Iran repeatedly violates the UN arms embargo which has been in place for over a decade."
US officials said the guided missile destroyer USS Forrest Sherman detained a small boat and personnel boarded the vessel, where the missile parts were found. The crew on the boat were transferred to the Yemeni Coastguard and the missile parts are in the possession of the United States, the officials said.
The weapons are still on board the US ship.
The US has consistently accused Iran of smuggling arms to Houthi rebels fighting the Yemeni government and has previously seized smaller and less sophisticated weapons in transit.
Smuggling weapons into Yemen is a breach of a UN Security Council resolution.
In a letter circulated on Wednesday, ambassadors from the UK, France and Germany urged UN Secretary General Antonio Guterres to tell the council that Iran’s ballistic missile activity is inconsistent with a council resolution endorsing the 2015 nuclear deal with Iran.
US President Donald Trump withdrew from the nuclear agreement in May last year. But it is still supported by the five other parties – France, Britain, Russia and China, all of which have power of veto in the Security Council, and Germany.
The letter talks of an Iranian missile “technically capable of delivering a nuclear weapon”.
French President Emmanuel Macron in November expressed concern over Iran’s nuclear weapons development, refusing to rule out further sanctions over its ballistic missile programme.
The developments came as US Secretary of State Mike Pompeo met Israeli Prime Minister Benjamin Netanyahu.
“The first subject I will raise is Iran, the second subject is Iran and so is the third,” Mr Netanyahu said, standing alongside Mr Pompeo before their meeting. “Iran is increasing its aggression as we speak. We are actively engaging in countering that aggression.”
Earlier, Mr Netanyahu said US sanctions were creating political and economic problems for Iran, and that while he had given up on European countries joining in the campaign, there was still a “wide spectrum” of options for increasing the pressure.
Mr Pompeo expressed support for the recent protests in Iran.
“These are people who are seeking freedom and a reasonable way to live, and they recognise the threat that’s posed by the kleptocrats who are running Iran," he said.
Mr Netanyahu last year welcomed the US withdrawal from the nuclear deal signed by world powers and Iran, and subsequent American sanctions.
The sanctions, against Iran’s vital oil sector, have hit the Iranian economy hard.
In recent weeks, Iranian forces have reportedly killed more than 200 people in demonstrations against rising fuel prices.
Protests have also rocked pro-Iran governments in Lebanon and Iraq, forcing leaders in both countries to resign.
“We’re seeing the Iranian empire totter," Mr Netanyahu said. “It’s important to increase this pressure against Iranian aggression.”