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Calling for an end to the escalating battle in the Marib region, Lenderking pointed a finger at Iran and its role in providing training to the Houthis and helping to "fine tune" their missile and drone programmes, Reuters reported.

"Unfortunately all of this is working to very strong effects as we see more and more attacks on the Kingdom of Saudi Arabia - and potentially other countries - more accuracy and more lethality. So this is a great concern to us," he said during a House of Representatives Foreign Affairs Committee hearing.

"Iran's support to the Houthis is quite significant, and it's lethal. We would welcome Iran playing a constructive role, if they are willing to do so. We have not seen any indication of that."

A spokesman for Iran's mission to the United Nations in New York said Lenderking's claims were unsubstantiated.

"In contrast, the US has been providing the deadliest weapons to those who are using them to kill innocent men, women and children on a daily basis," the spokesman said, referring to the US' role in providing weapons to Saudi Arabia in its military campaign against the Houthis.

In February the rebels renewed their offensive on the oil-rich province of Marib - a stronghold held by the internationally-recognised government.

The attack on Marib stirred up violence in other areas, including government-held Taiz province, which is besieged by the Houthis. Clashes also took place in the province of Hajjah and the port city of Hodeida.

This escalation threatens to worsen the already severe humanitarian crisis in Yemen, as the province is sheltering around a million people who have fled Houthi offensives elsewhere in the country.

Saudi Arabia and the Biden administration have recently offered separate ceasefire proposals. The Houthis, however, turned them down.

The conflict in Yemen began with the 2014 takeover of the capital Sanaa by the Houthis. A Saudi-led coalition allied with the exiled President Abed Rabbo Mansour Hadi has been fighting the rebels since March 2015.

The war in Yemen has spawned the world's worst humanitarian crisis, leaving millions suffering from food and medical shortages.

It has killed some 130,000 people, including fighters and civilians, according to a database project that tracks the violence.

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