In the mean time, Yemenis fleeing the violence find themselves trapped between these different fronts, and, with ports closed and air traffic restricted, there are few avenues of escape.
Quite how many are in need or where they are, however, remains unclear.
The UN and aid agencies have evacuated almost all their foreign staff, while those who remain are often unable to travel to assess the humanitarian situation because of the widespread insecurity.
It’s the same story for the number of displaced. The UN estimates around 100,000, but other experts say the real number could be far higher. Yemen was host to more than 250,000 refugees and one million migrants even before the current conflict.
In some parts of the country, reliable information on the location of those in need is almost non-existent, said Haajir Maalim, country director at Action Against Hunger (known by its French acronym ACF). The UN and NGOs are trying to carry out an assessment but many desperate people may simply fall under the radar, he added.
What is clear is that no one is doing enough to protect civilians. In one particularly disturbing incident, an established camp for the displaced in northern Yemen was bombed – allegedly by the Saudi-led coalition – causing around 600 families to flee after dozens of civilians, including several women and children, were killed.
3. Aid delivery increasingly difficult
In almost every way possible, aid is being prevented from reaching the people who need it most.
Firstly, the violence itself is stopping aid organisations from doing their work. In Aden, fighting is so intense that people cannot even get the wounded to hospital.
Marie-Elisabeth Ingres, head of mission for Médecins Sans Frontières (MSF) in Yemen, said their hospital in Aden had initially been receiving around 50 wounded people a day.
As fighting has intensified, however, that number has decreased rapidly. It is just too unsafe for people to move around, and ambulances, themselves increasingly targeted, are unable to reach the wounded.
“The last few days we have had very few people,” Ingres told IRIN on Thursday.
With most ports either under the control of Houthi rebels, directly involved in the fighting, or blockaded by the Saudi-led coalition, it is hard to get aid into Yemen by sea.
On Thursday, a letter was widely shared on social media purporting to show the agents of a ship carrying wheat complaining that it had been blocked by warships from entering port.
Many international shipping lines are now abandoning deliveries altogether.
4. Shortages of food, petrol, clean water, medical supplies
The shipping problem raises the prospect of widespread food shortages, especially as more than 90 percent of Yemen’s key grains are imported.
Despite the lack of money, the cost of goods is soaring. In Aden, for example, food prices have already risen by around 20 percent, according to the UN.
But food is only one of many essentials that civilians in Yemen fear could be running out.
“Because of acute shortage of oil we are not able to provide as much [aid],” said ACF’s Maalim. “Our teams are incapacitated. Even in areas where access is still possible, there is no fuel.”
Aid organisations are also concerned they will have a limited ability to respond if clean water and medical supplies run low.
“Water is a problem with [displaced people] in the south specifically,” Maalim said. “The fear is that if nothing is done in the coming days… there could be an outbreak of cholera, malaria and other diseases.”
Many foreign health workers, on which the system relied heavily, have already fled the violence.
“We don’t have enough medical staff, [let alone] qualified ones,” MSF’s Ingres said. “We need nurses.”
Without an unlikely return to negotiations, there is little hope of avoiding a large-scale humanitarian crisis.
Ban on Thursday reiterated his call on all parties to rejoin UN-brokered negotiations to forge an end to the crisis.
“The last thing the region and our world need is more of the chaos and crimes we have seen in Libya and Syria,” he said.