The Saudi-led coalition fighting in Yemen has redeployed troops to the Marib region and increased air strikes to try to repel an advance by the Iran-aligned Houthi group, sources in the military and the internationally recognised government said.
Fighting in the gas-producing region has escalated as the Houthis try to take Marib city, the government's last stronghold in the north of Yemen.
The fighting threatens to displace hundreds of thousands and complicate renewed U.S. and U.N. efforts to find a political solution to the more than six-year-old civil war in Yemen.
A government source in Marib and a military source told Reuters that hundreds of fighters had arrived from Hadhramout and Shabwa in the south, where the Saudi-backed government is based, and from the suburbs of northern Sanaa province.
One resident said military reinforcements passed through Marib on Thursday and that coalition warplanes had carried out several air strikes.
Clashes could be heard from the frontline about 30 km (18 miles) to the west of the city, the resident added.
The Yemeni army said in a Twitter post on Thursday that coalition air raids had destroyed a Houthi missile launching pad in Makhdara, west of Marib city, and four ballistic missiles.
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The United States urged the Houthis on Tuesday to halt their offensive and join international efforts to end the conflict, widely seen in the region as a proxy war between Saudi Arabia and Iran.
Echoing the U.S. appeal, U.N. Yemen mediator Martin Griffiths said on Thursday "the quest for territorial gain by force threatens all of the prospects of the peace process."
The Houthis ousted the government from power in the capital, Sanaa, in late 2014, prompting the Saudi-led coalition to intervene months later. The Houthis deny being puppets of Iran and say they are fighting a corrupt system.
On Thursday, the Houthi-run Al Masirah channel said the movement's interior minister had ordered a wide deployment of security forces and rescue workers in Marib "to match military victories with security victories by protecting citizens".
Around 80% of Yemen's population relies on aid and millions are on the verge of famine.