Iranian-supplied and led Houthi rebels in Yemen have started an offensive against the oil-rich Marib province located about 75 miles east of the rebel-held capital of Sanaa.

The offensive by the Houthis, if successful, would be a huge blow to the internationally recognized government of Yemen, and deprive it of its last stronghold in the northern part of the country. 

Mark Lowcock, the UN undersecretary-general for humanitarian affairs, posted on Twitter that an assault on Marib could endanger two million civilians and cause hundreds of thousands to flee the city, which would have “unimaginable humanitarian consequences.”

“Now is the time to de-escalate, not to add even more to the misery of the Yemeni people,” Lowcock added.

Yemeni military officials said to the Agence France-Presse (AFP) that the Houthis had advanced along two axes toward the city amid heavy fighting. The number of troops killed on both sides is expected to be in the hundreds. 

“The rebels have advanced north and west of the city… and tightened their grip on hills overlooking supply lines for several fronts,” one military source said to AFP.

Al Masirah, the Houthi television mouthpiece said that Saudi aircraft conducted 13 airstrikes, 11 on locations in the district of Sirwah and two in the district of Madghal. Marib had been mostly spared the violence that has plagued the country which is why it hosts nearly one million internally displaced persons. 

The United States called on the Houthis to stop the attack on Marib, cease all military operations, end cross-border strikes on Saudi Arabia and participate in an UN-led peace process. The attacks on Saudi Arabia have targeted civilian infrastructure and the Abha international airport according to the Saudis.