Nearly 50,000 people have fled a sudden advance by Houthi forces in western Yemen, with entire villages emptied and families trapped near frontline fighting with depleting food supplies, according to aid workers.

Civilians in the Middle East’s poorest country have found themselves victims of a global struggle for control over one of the most important shipping routes in the world, which threatens to turn Yemen’s on-off conflict into a forever war.

In a lightning advance this week, the Iran-backed Houthi militants have seized a string of towns along the Red Sea coast from forces aligned with the US- and Saudi-backed Yemeni government. Hundreds of people are reported to have been killed. Many fear a return to the full-scale civil war that killed 150,000 and brought many others to the brink of famine.

The UN International Organization for Migration (IOM) said on Friday that 46,000 people had been displaced across the governates of Taiz and Hodeidah, where steep verdant mountain ridges give way to arid coastal plains. For many families, this was their second or even third time they had been forced out of their homes.

A significant number of people were observed to be leaving the city with their belongings. Fighting has cut key roads, while phone lines are down in some areas. “The scale of displacement is increasing rapidly,” said Joe Lowry, IOM’s Middle East and north Africa spokesperson in Cairo.

The most urgent needs were emergency shelters, cash assistance, food, drinking water, household items, healthcare and protection services. “Civilians are bearing the brunt of renewed frontline warfare, airstrikes, shelling, and large-scale displacement,” Lowry said.

“Many of those fleeing have arrived with very few belongings after leaving suddenly, due to shelling and frontline advances.”

Lowry said aid workers were having to constantly reassess access, and adapt movements, as the situation changed. “It’s a highly fluid and uncertain situation marked by rapid territorial changes, population movements and growing concern among communities about further escalation,” he said.

The Houthi advance has come unexpectedly and fast, particularly the fall of the strategic port city of Mocha – widely regarded as the birthplace of the global coffee trade and the eponym​ for chocolate-flavoured coffee.

Mohamed, 46, who left Mocha in the middle of the night, told Agence France-Presse there was an “apocalyptic” atmosphere in the city. Another man, Mujahid Tahami, who set off for the southern government-held city of Aden, said some families had fallen victim to “looting by gangs as they tried to flee”.

Others reported that they had managed to flee in time, some on motorbikes, but other family members were not so lucky and were stuck.

Yemen analysts have long criticised the US and its allies for taking a security-focused approach to the country that undervalues diplomacy and development. Washington launched a campaign of drone attacks against Sunni al-Qaida militants for much of the 21st century, but did not pay significant attention to the hardline Shia Houthi insurgency until its forces claimed the capital in 2014.

By taking control of key areas this week, including islands off the coast, the anti-west Houthis hope to gain a direct presence near the Bab al-Mandab strait, a 20-mile stretch of water and vital trade and energy route that connects Asia with Europe. ​

The strait’s name is roughly translated as “the gate of tears” or “the gate of grief”, and its strong currents and winds made the passage between the Arabian peninsula and Africa’s east coast hazardous for centuries. Modern-day cargo ships have no physical issue passing through.

​But if ​the Houthis were to close the natural chokepoint, the impact on the global economy could be disastrous. Ships would have to reroute around the southern tip of Africa, adding thousands of extra nautical miles. The fighting has already contributed to the price of oil rising above $100 a barrel after weeks when it was trading closer to $80.

The fighting in Yemen, as well as Houthi missile strikes on Saudi Arabia, is the most significant violence in the country since the UN-brokered peace plan in 2022.

The UN special envoy for Yemen, Hans Grundberg, told an emergency security council meeting on Thursday that the country faced “a new and more dangerous phase of war”.

“This renewed war should alarm us all,” Grundberg said. “It effectively marks the end of the relative calm that Yemen has experienced for over four years since the UN-brokered truce. It not only threatens to plunge Yemen into a prolonged and expanding war, but also risks yet again dragging the country into the wider and unresolved regional confrontation, with global repercussions.”

Instability in Yemen has been exploited by regional powers such as Saudi Arabia, Iran and the United Arab Emirates to fight proxy battles, and the failure to end the violence has led to the growth of powerful militias. The US-Israeli war on Iran has only heightened the tensions, with regional animosity at an all-time high.

This has compounded one of the world’s worst humanitarian crises. The most recent UN humanitarian report, released last month, warned that the country of 40 million was facing a deepening malnutrition crisis, with roughly half the population facing acute food insecurity nationwide.

“Years of conflict, economic deterioration, displacement, climate-related shocks and regional tensions have steadily weakened livelihoods and household resilience,” the report warned. “At the same time, declining humanitarian funding and operational constraints are limiting the ability of aid organisations to provide assistance at the scale required.”

On the Red Sea coast, those who escaped the latest wave of fighting had their lives, but were left in shock.

“It’s a catastrophe, and the attacks are ongoing,” said Mohammed Khudairi, who fled the fighting to a displacement camp of wood-and-straw huts near the areas of Khokha. “We managed to leave, but others remain trapped there.”