Intense fighting has broken out between former allies in a battle for control of Yemen’s interim capital, Aden, threatening to open a new front in the war and raising fears for civilians’ safety.

Forces loyal to the Yemeni government were hit by airstrikes on Thursday as they travelled towards the southern city, killing at least 30 troops, a government commander said. He did not say who carried out the strikes, but a coalition led by Saudi Arabia and the United Arab Emirates is the only party to the four-year-old conflict with airpower.

The attack is a major escalation in a three-week struggle for Aden between the Yemeni government and the separatist Southern Transitional Council (STC), which is backed by the UAE.

Yemen’s deputy foreign minister, Mohammed Abdullah al-Hadram, said in a tweet that the Yemeni government held the UAE responsible for the “explicit extra-judicial targeting”, adding that the strikes had also killed several civilians.

Officials in the UAE declined to comment.

The government and the STC are ostensibly allies in the Saudi-led coalition fighting against Houthi rebels who seized much of north Yemen and the capital, Sana’a, in 2014. The STC’s ultimate goal, however, is independence for the south from what it says is corruption and mismanagement by the government, which it says has ties to Islamists.

The rift has also exposed tensions between the coalition members Saudi Arabia, which backs the government of President Abd-Rabbu Mansour Hadi, and the UAE, which funds and arms the STC and allied militias such as the Security Belt. The UAE has fallen out with Hadi’s side as it includes a party the UAE sees as close to the Muslim Brotherhood, which the Gulf state seeks to neutralise across the Middle East.

While both countries reiterated their support for the floundering coalition on Monday, worries are growing over how the new fighting will affect a country where 80% of the population – more than 24 million people – rely on aid to survive and 10 million people are on the brink of starvation.

“We were scared that the fighting in Aden would continue for days, but today it has stopped in the city at least,” said Jaclin al-Batani, a local university lecturer.

Médecins Sans Frontières said its teams in Aden admitted 51 people in the space of a few hours on Wednesday, 10 of whom were dead on arrival.

“It’s total chaos here,” said Caroline Seguin, MSF’s programme manager for Yemen, who was in Aden. “There was fighting in the city all day yesterday. Things appear to have calmed down a bit this morning but we expect the hostilities to resume at any point.”

The Security Belt militia seized Aden as well as other cities in the south including Ataq and Zinjibar in three days of fierce fighting that left dozens of people dead this month. Hadi’s government described the offensive as a coup.

On Wednesday militias loyal to Hadi said they had regained control of Aden, including the airport and the presidential palace, after a huge push to drive out the separatist fighters.

But on Thursday government troops appeared to have withdrawn to neighbouring Abyan province. The Security Belt had regained full control, said an STC spokesperson, Haithan Nezar.

“To whomever said the Southern Resistance has fled, I say: We are here,” STC vice-president Hani Ben Brik said in a video shared on social media which showed him with dozens of fighters outside Aden’s airport.

The newest chapter in Yemen’s complex civil war was triggered by the UAE’s announcement in July that it would withdraw thousands of troops from the country, shrinking its involvement in the fight against the Iran-backed Houthis.

The UAE has denied the timing was linked to rising tensions between the US and Iran over Tehran’s collapsing nuclear deal with world powers.