Fighters loyal to Yemen's Saudi-backed government take part in a parade during a graduation ceremony of officers and the completion of specialized military courses, in the country's northeastern province of Marib, on March 17, 2023. (PHOTO / AFP)

ADEN - A flare-up of fighting in Yemen's energy-rich Marib province is testing 10 months of relative calm, the longest stretch in an eight-year civil war, under a UN-brokered truce deal that lapsed in October but had largely held.

Forces of the Iran-aligned Houthi movement, de facto authorities in northern Yemen, launched an attack late on Tuesday into Marib's Harib district, government and military sources and a resident told Reuters.

Marib is the last stronghold in northern Yemen of the internationally recognized government and is the country's sole gas producing region with a large oilfield

The three sources said there was an unknown number of casualties in the clashes.

A spokesman for the Houthi group did not immediately respond to a request for comment on the escalation.

Marib, which lies around 120 km (75 miles) east of the capital Sanaa, had been the main frontline before the truce was first agreed last April, as forces of a Saudi-led military coalition repelled advances by the Houthi movement bent on seizing full control of the province in central Yemen.

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Yemen has been mired in violence since the Houthis ousted the government from Sanaa in late 2014, prompting the Saudi-led coalition to intervene months later.

Marib is the last stronghold in northern Yemen of the internationally recognized government and is the country's sole gas producing region with a large oilfield.

The escalation comes at a time Saudi Arabia and the Houthi movement have been holding direct talks in parallel with United Nations efforts to reinstate and expand the truce deal.

UN special envoy Hans Grundberg earlier this week voiced hope things were moving "in the right direction", citing recent developments including a prisoner swap between the warring sides and an agreement between Saudi Arabia and Iran to restore bilateral ties.

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The conflict has killed tens of thousands of people, wrecked Yemen's economy and pushed millions into hunger.