The United Nations special envoy to Yemen has called for immediate de-escalation as a drastic surge in fighting threatens to exacerbate the country's long-running war and further complicate a fragile peace process.

The renewed violence between a Saudi-UAE-led military coalition and Houthi rebels has reportedly caused hundreds of casualties since it erupted almost two weeks ago. The coalition backing the internationally recognised government has stepped up air raids on rebel targets northeast of the capital, Sanaa, following a period of relative calm, while the Houthis targeted government-held areas.

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"The parties in Yemen must de-escalate violence and renew their commitment to a peaceful resolution of the conflict," UN envoy Martin Griffiths wrote on Monday on Twitter.

"The Yemeni people deserve better than a life of perpetual war," he said.

Griffiths said the warring sides must fulfil promises and "build a conducive environment for the peace process".

The UN Security Council called emergency consultations for Tuesday at the United Kingdom's request following the escalation of violence. Karen Pierce, the UK's ambassador at the world body, said the council would receive a closed-door video briefing from Griffiths.

Houthis gaining ground

The warring factions have concentrated their forces in three main areas: Nehm, a half-hour drive from Sanaa; Jawf, a mountainous northern district; and Marib, a western province that saw one of the deadliest incidents earlier this month when a missile attack on a government military camp killed more than 100 people.

Fighting this week was the most intense those provinces had seen in three years, according to observers.

A wave of more than 40 coalition air raids hit rebel targets, destroying many of their tanks and armoured vehicles, Houthi officials told The Associated Press news agency. The officials spoke on condition of anonymity in line with regulations.

Despite heavy losses on both sides, the Houthis are gaining ground, officials told the AP. Rebels seized a key supply line linking Marib with Jawf and were approaching the capital of the northwestern province.

Fighting also flared up on Monday in the large government-controlled city of Taiz, where a mortar shell fired by Houthis struck a busy market, killing three civilians and wounding 10, according to the AP. Meanwhile, heavy clashes in the central province of Bayda killed 13 fighters on both sides.

For months, back-channel negotiations in Oman between Saudi Arabia and the Houthis stirred modest hopes for reconciliation. But sharply escalating violence has put the political process on shaky ground.

Peter Salisbury, the Yemen expert at the International Crisis Group (ICG), said the Houthis may be using their military successes to gain leverage before talks resume next week in Oman.

"Both sides seem to want some sort of truce," Salisbury said. "But the danger is that if the Houthis feel they're on the front foot, they'll keep pressing advances and that will make these negotiations very difficult."

In a report published on Friday, the ICG said the Houthis "appeared to be making the biggest gains on the battlefield".

The think-tank warned that if the renewed fighting spread, it would be "a devastating blow to current efforts to end the war".

Yemen's conflict has left tens of thousands of people dead, mainly civilians, and triggered what the UN has said is the world's worst humanitarian crisis.