At least 50 Houthis and pro-government troops have been killed as fighting intensifies for Yemen's key city of Marib, military sources said on Sunday.

“In the past 48 hours, 43 Houthi fighters were killed mostly in coalition air strikes” west of Marib, a military source told AFP, while another source said at least seven loyalists died in fighting.

About 400 deaths have been reported during clashes in Marib this month, after a lull in fighting in the region.

The Houthis rarely report casualty numbers, but figures were confirmed by medical sources.

According to the military sources, who spoke on condition of anonymity, the Houthis have made advances and seized four districts — one in Marib and three in Shabwa.

“Three districts in Shabwa have fallen in limited clashes and within hours,” one official told AFP.

Hundreds of people have died this month alone after the Iran-backed Houthis renewed their assault on Marib, the government's last stronghold in the oil-rich north.

Control of the oil-rich region would strengthen the Houthis' bargaining position in peace talks.

The Saudi-led military coalition intervened in Yemen in 2015, backing forces of the ousted government of President Abdrabu Mansur Hadi against the Iran-aligned Houthis.

The Houthis initially escalated their efforts to seize Marib in February, hoping to seize control of the region's oil resources.

-Agencies