The U.S. is imposing sanctions on two Houthi rebel commanders who are leading the Iran-backed group’s offensive on Yemen’s oil-rich Marib province, Timothy Lenderking, the U.S. special envoy for Yemen, said Thursday.

Lenderking didn’t describe in detail what sort of penalties would be imposed against Muhammad Abd al-Karim al-Ghamari, head of the general staff of the Houthi militia forces who is leading the campaign, and Yusuf al-Madani, commander of the military zone that includes the hotly contested port city of Hodeidah. Marib is connected to Hodeidah by an oil pipeline.

“We’ve been troubled by the fact that the Houthis continue to fight in Marib,” Lenderking said in a call with reporters. “Marib, despite their predictions, did not fall during the month of Ramadan. It’s not falling now and it’s not going to fall anytime in the foreseeable future. So the Houthis aren’t winning in Marib and instead they are putting a great deal of stress on an already very fragile humanitarian situation.”

He urged the group to engage with ceasefire efforts and said the U.S. supports opening all ports and airports in Yemen for commerce and for the humanitarian aid.

Lenderking was appointed shortly after President Joseph Biden came into office and tasked with helping end Yemen’s seven-year-old war, which has splintered into domestic fronts and a broader conflict with neighboring Saudi Arabia. The Houthis have been fighting Yemen’s United Nations-recognized government since 2014. A Saudi-led coalition intervened the following year on the side of the government and has since faced repeated drone and missile attacks from Yemen on its energy infrastructure and defense facilities. The UN has called the conflict -- in which tens of thousands of people have died -- the world’s worst humanitarian crisis.

The sanctions announcement came as Elizabeth Warren and 15 other Democratic senators asked Biden to use all diplomatic tools, including leveraging weapon sales and military cooperation, to end Saudi Arabia’s blockade of Yemen, which has been blamed for exacerbating the humanitarian suffering.

Lenderking said Baghdad-brokered talks that have been taking place between Saudi Arabia and Iran are a “potentially constructive engagement” that could ease tensions across the region and would have positive impact on Yemen.

Saudi Arabia has also sought to end its involvement in Yemen as attacks on its own territory have occasionally hit key energy facilities and shaken oil markets, proposing in March a comprehensive cease-fire to begin once Houthi fighters accept the initiative. The group hasn’t accepted the plan and Lenderking accused it of moving the goalposts.