The United States on Thursday announced sanctions against two of Yemen's Houthi rebel commanders, who are leading an offensive to seize the last northern government stronghold of Marib.
Yemen's civil war, which started in 2014, pits Iran-backed Houthi rebels against an internationally recognised government supported by a Saudi-led military coalition.
The Houthis launched in February a fierce offensive to seize Marib, capital of an oil-rich region, where heavy battles have exacerbated what the UN already calls the world's worst humanitarian crisis.
US Special Envoy for Yemen Tim Lenderking said sanctions were placed on Mohammed Abdul Kareem al-Ghamari, the Houthi's Chief of the General Staff, who is leading the push to seize Marib, as well as Youssef al-Madani, another Houthi commander leading forces in the campaign.
"If there were no offensive, if there were a commitment to peace, if the parties are all showing up to deal constructively with the UN envoy, there would be no need for designations," Lenderking told reporters in an online press briefing.
Lenderking said he regretted that the Houthis were absent during recent ceasefire talks.
"The Houthis are not winning in Marib and, instead, they're putting a great deal of stress on an already very fragile humanitarian situation," he added.
Yemen's long war has claimed tens of thousands of lives and displaced millions.
Marib lies about 120 kilometres (75 miles) east of the rebel-held capital Sanaa.
Along with its surrounding oil fields, it makes up the last significant pocket of government-held territory in the north, the rest of which is under rebel control.