"The clashes taking place in al-Dhalea are less visible than those in along Yemen's west coast (where Hodeidah is) but threaten to have a similarly disastrous impact," said Suze van Meegen of the Norwegian Refugee Council.
"It is absolutely critical that supplies coming through Aden move inland as quickly as possible, keeping commercial markets afloat and replenishing humanitarian supplies."
The Saudi-backed coalition of pro-government forces and their Iran-aligned Houthi enemies agreed a ceasefire and troop withdrawal in Hodeidah in December, in the first major diplomatic breakthrough in a conflict that broke out in late 2014.
That deal averted an all-out assault on the port that risked unleashing famine but, while the fragile peace is largely holding, the troop pullback has stalled and thousands of tonnes of grain - inaccessible by aid agencies until Sunday - have started to rot.
Meanwhile, in other areas, ground fighting has intensified and air strikes continue.
Al-Dhalea straddles the main roads between the government-controlled southern port of Aden, a major entry point for aid and commercial supplies, and the Houthi-controlled capital Sanaa.
Heavy clashes and artillery in al-Dhalea are forcing aid, commercial traffic and civilians to divert onto long and dangerous mountain passes, reducing humanitarian access and increasing journey time and costs.
"Since November I think we are on our fourth alternative route as things (hostilities) expand," said Frank McManus of the International Rescue Committee (IRC) aid organisation.
Two weeks ago a bridge connecting al-Dhalea to neighbouring Ibb was blown up, severing that route. Local reports blamed the incident on the Houthis after they stormed the area, though responsibility has not been confirmed.
Heavy fighting has also broken out in the last couple of months in northwestern regions Hajjah and Abbs, and Taiz in the southwest, causing deaths and displacing hundreds of thousands.
The IRC said the al-Dhalea clashes have forced it to suspend and relocate health clinics, cholera treatment, education and other services in recent weeks.
Yemeni authorities have reported more than 200,000 suspected cases of cholera since January.
A UN Development Programme report said that if the conflict were to end this year, it would have killed around 233,000, with 56 percent of those being indirect deaths from lack of food, health services and infrastructure.
Before the Hodeidah agreement, the al-Dhalea front between the Houthis and government forces was static.
"(Now it's) an active front line with people being displaced on a regular basis and with the expectation it (fighting) will continue in that area," McManus said.
Deep mistrust between the warring parties has stalled UN efforts to implement the Hodeidah deal, with which local security forces will run the port after withdrawal a key sticking point.
The UN is optimistic a troop withdrawal can happen soon, but diplomats have told Reuters a breakthrough remains distant.
Government coalition allies Saudi Arabia, the United Arab Emirates, the United States and Britain last week repeated calls for the Houthis to redeploy from Hodeidah. They said they would be looking for the U.N. security council to review progress on May 15 "with the expectation that implementation will be under way at that point".
The head the Houthis' Supreme Revolutionary Committee, Mohammed Ali al-Houthi, told Reuters that, while discussion points remained, he hoped troop redeployment would take place soon.
Houthi forces drove the government of President Abd-Rabbu Mansour Hadi out of Sanaa in late 2014 and the Saudi-backed coalition of Yemeni and Arab forces intervened in March 2015 to restore it.
The Houthi movement, which says it is a revolution against corruption, controls Sanaa and most population centres.
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