ADEN (Reuters) - Hundreds of fighters have been killed in a weeks-long Houthi offensive on the Yemeni region of Marib, military sources and a local official said on Monday, in the conflict’s most deadly clashes since 2018. The attack by Iran-aligned Houthi forces on government-held Marib city comes amid renewed diplomacy to end the six-year war, and as the United States said it would end support for a Saudi-led coalition backing the internationally recognised Yemeni government. The United Nations has urged the Houthis to return to negotiations, and said the offensive threatened mass displacement. Hundreds of fighters from both sides have been killed in clashes in the gas-rich Marib region, the sources said. They were not authorised to speak publicly about operational matters. The sources added that the Houthis may have lost more fighters than the government during the offensive, with coalition forces boasting air supremacy. “It is a blood bath,” said one military source. Two sources said the majority of casualties were fighters, not civilians. Pro-government defences in Sarwah, to Marib’s west, have collapsed, with the frontline now roughly 20 km (12.4 miles) from the city, according to the sources. The death toll from the battles of the last three weeks are on a scale not seen since 2018, when Saudi-led coalition forces launched an offensive in the Red Sea coastal area to take Yemen’s biggest port of Hodeidah. That attack was stopped amid humanitarian concerns. Slideshow ( 3 images ) Mass funerals for Houthi fighters killed in the Marib operation have been held over the past week in the Houthi-controlled capital Sanaa. Houthi spokesman Mohammed Abdulsalam said on Twitter on Sunday their forces would keep fighting until the entire country is “liberated”. “What Marib is witnessing is part of a national liberation battle, in the face of the continued aggression and blockade,” Abdulsalam said. Abdulsalam and a spokesman for the government of President Abd-Rabbu Mansour Hadi did not respond to requests for comment on the reported casualties. Yemen’s frontlines have moved little in recent years, but the war has shifted in the past 12 months to Marib, the last stronghold of Hadi’s loyalists. Around 116,000 people have already left their homes over the last year, and thousands more have been uprooted in recent weeks, the U.N.’s International Organization for Migration (IOM) said. The escalation comes amid renewed efforts to end the war after a dramatic change in the U.S. position towards Saudi Arabia, which leads a military coalition hoping to restore Hadi’s government. The new U.S. administration of President Joe Biden revoked a designation of the Houthis as a foreign terrorist organization, imposed by former President Donald Trump, and halted support for offensive operations by the Saudi-led coalition. Slideshow ( 3 images ) The United States and the U.N. have called on the Houthis to halt the Marib attack, cease all military operations, end cross-border strikes on Saudi Arabia and participate in a U.N.-led peace process. “The Houthis are clearly taking advantage of Saudi Arabia’s isolation after the election of Joe Biden... and its disintegrating coalition,” said a Western diplomat. “It is a big gamble but I also wonder what do they have to lose?”