Over 45,000 people are fleeing their homes as Houthi forces push to usurp Marib, the city that has seen around 1 million people flee since late 2014.
Over 150,000 people will be killed in fighting alone in Yemen this year, the UN agency says while an estimated 1.3 million people in total will have died by 2030.
The Houthis have detained Yemeni staff in the US compound in a daring attack, which might be a message to Washington that they are in charge in the capital.
Coalition operations took place in Sirwah to the east of Marib city and Al Bayda province to the south.
The capture of the strategic oil-rich city of Marib would aid the Houthis' expansion to other provinces, marking a major blow to the coalition.
At least 200 fighters were killed and hundreds others wounded in clashes between government forces and Houthi rebels as battle for the strategic city of Marib rages in the war-torn country.
The Houthis have made territorial gains in Marib despite coalition airstrikes and fierce battles inflicting damages on both sides.
The coalition said it destroyed 36 military vehicles and killed more than 264 rebel fighters in the past 72 hours, the Saudi Press Agency reported.
Iran-backed Houthis were now in control of the Assilan, Bayhan and Ain districts in Shabwa as well as the Abdiya and Harib districts in Marib, said local officials and residents.
Hundreds of Iran-backed rebels have been killed in the battle for Marib in recent days, says the Saudi-led coalition backing the Aden government.
More than 156 Houthi rebels were killed and eight military vehicles were destroyed in an operation in Abdiya, south of Marib, says the Saudi-led coalition fighting the militants in Yemen.
The intensified fighting for the northern city leaves at least 50 Houthi rebels and Yemeni pro-government troops dead. The continuing clashes have reportedly claimed around 400 lives in September.
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