Iran has violated a UN arms embargo by failing to block supplies to Yemen's Huthi rebels of ballistic missiles that were fired at Saudi Arabia, a UN panel of experts says, bolstering US and Saudi claims of Tehran's military involvement in Yemen.

The report seen by AFP on Friday does not identify the supplier but makes clear that missile debris inspected by the experts were of Iranian origin."The panel has identified missile remnants, related military equipment and military unmanned aerial vehicles that are of Iranian origin and were introduced into Yemen after the imposition of the targeted arms embargo" in 2015, said the report to the Security Council.

"As a result, the panel finds that the Islamic Republic of Iran is in non-compliance with paragraph 14 of resolution 2216" that imposed the ban on arms sales to Yemen, said the 79-page report presented on Tuesday.

Iran has strongly denied arming the Huthis and last month accused US Ambassador Nikki Haley of presenting "fabricated" evidence that a November 4 missile fired at Riyadh airport was Iranian-made.

Haley told the Security Council in December that the United States will push for action against Iran over the missile attacks targeting its ally, but Russia quickly signaled that it would not endorse such plans.

The missile attack on Riyadh airport "changed the tenor of the conflict, and has the potential to turn a local conflict into a broader regional one," the experts warned.

The panel also said it was investigating whether Iran had sent "advisers" to help the Huthis in their war against the Saudi-led coalition.

- Missile debris -

The UN experts traveled to Saudi Arabia in November and again last month to inspect the remnants of missiles fired by the Huthis in May, July, November and December.

They found that design features of the missile debris were "consistent with those of the Iranian-designed and manufactured Qiam-1 missile" and "almost certainly produced by the same manufacturer."

The drones were "virtually identical in design" to that of an Iranian-made UAV manufactured by the Iranian Aircraft Manufacturing Industries (Hesa), said the report.

Iran "failed to take the necessary measures to prevent the direct or indirect supply, sale or transfer" to the Huthis of the missiles, storage tanks for propellant and drones, it concluded.

A separate report last month said UN officials had examined the missile fragments and found that they were of "common origin" but they were unable to reach any firm conclusions about whether Iran was the source.

Already one of the Arab world's poorest countries, Yemen has been brought to its knees since the Saudi-led coalition intervened in March 2015 in support of the government.

More than 8,750 people have died in the war and the country is facing what the United Nations has described as the world's worst humanitarian crisis.

The experts criticized both the coalition for killing civilians with airstrikes and the Huthi forces for attacks that have overwhelmingly affected non-combatants.

"The panel has seen no evidence to suggest that appropriate measures were taken by any side to mitigate the devastating impact of these attacks on the civilian population," the report said.

"After nearly three years of conflict, Yemen as a state has all but ceased to exist.

"Instead of a single state, there are warring statelets, and no one side has the political support or the military strength to reunite the country or achieve victory on the battlefield."