Iran’s support for Yemen’s Houthi movement is “quite significant and it’s lethal,” US special envoy on Yemen Tim Lenderking said on Wednesday, as he called a battle for Yemen’s gas-rich Marib region the single biggest threat to peace efforts, Reuters reports.

Lenderking told US lawmakers that Iran supports the Houthis in several ways including through training, providing lethal support and helping them “fine tune” their drone and missile programs.

“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,” Lenderking told a House of Representatives Foreign Affairs Committee hearing.

“Iran’s support to the Houthis is quite significant, and it’s lethal,” Lenderking said.

A Saudi Arabia-led military coalition intervened in Yemen in 2015 after the Iran-aligned Houthi group ousted the country’s government from the capital Sanaa. The Houthis have said they are fighting a corrupt system.

“We would welcome Iran playing a constructive role, if they are willing to do so. We have not seen any indication of that,” Lenderking said.

Iran has denied supporting the Houthis. A spokesman for Iran’s mission to the United Nations in New York dismissed Lenderking’s remarks as unsubstantiated claims against Iran.

“Iran has, time and again called for a peaceful settlement of the conflict in Yemen,” the Iranian spokesman said. “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.”

US President Joe Biden said in February that Washington was ending all American support for the Saudi-led offensive operations in Yemen, including relevant arms sales.

Since taking office in January, Biden has made Yemen a priority and appointed Lenderking to help revive stalled UN efforts to end a conflict widely seen as a proxy war between rivals Saudi Arabia and Iran.

“What I see is continued aiding and abetting an army of Houthis by the Iranians so that they can continue attacking Saudi Arabia, and unfortunately those attacks have risen quite strongly in the last couple of months,” Lenderking told a Senate Foreign Relations subcommittee hearing later on Wednesday.

The United States should “leave the matter of finding a resolution to the senseless conflict in the capable hands of the regional countries,” the Iranian UN mission spokesman said.

Fighting has intensified in recent days as the Houthis push their offensive to take Marib, which if successful would strengthen the movement’s hand in any future political negotiations.

“This offensive is the single biggest threat to peace efforts and is also having devastating humanitarian consequences. If we do not stop the fighting in Marib now, it will trigger a wave of even greater fighting and instability,” Lenderking said.

There are about 70,000 US citizens living in neighboring Saudi Arabia, Lenderking said, and “it is our greatest fear that Americans will be killed in a Houthi attack.”