Separatist fighters in southern Yemen who appeared to be on the brink of defeat on Wednesday recovered some lost ground on Thursday as the United Arab Emirates launched airstrikes to support them.

This direct intervention by the Emiratis in a conflict between two rival factions which are theoretically allies is sure to exacerbate tensions in the Saudi-led coalition against the Houthis who control much of northern Yemen.

For the last four weeks, forces linked to the separatist Southern Transitional Council (STC) have been fighting forces that support the "legitimate" government of President Abd Rabbu Mansour Hadi (background here).

The conflict has far-reaching implications because the southern forces are seen as proxies of the UAE – Saudi Arabia's main partner in the coalition. The pro-government forces, meanwhile, have close ties with the Saudis.

On Thursday the Yemeni government issued a strongly-worded statement accusing the UAE of carrying out airstrikes in and around Aden, and in Zinjibar, some 40km further east. "We hold the UAE fully responsible for this blatant assault which is inconsistent with law and international legal instruments," it said.

The bombing claim was initially viewed with scepticism by observers outside Yemen who doubted that the UAE would choose to escalate the situation in this way in view of the likely damage to Saudi-Emirati relations. 

Shortly afterwards, however, the UAE confirmed it had carried out "precise and direct air strikes" on Wednesday and Thursday against what it described as "terrorist militias". The statement continued:

"The military operation against the terrorist militias was based on confirmed field intelligence that the militias prepared to target the coalition forces – a development which required a preemptive operation to avert any military threat.

"The strikes against the Arab Coalition were launched by armed groups affiliated with terrorist organisations. These armed groups attacked the Arab Coalition at Aden Airport, causing two injuries to the coalition forces.

"Accordingly, the Coalition responded as per their right of self-defense to protect the security of their forces."

The strikes are reported to have killed at least 30 people and injured many more.

The pro-government forces contain Islamist elements and one of the claims made by the Emiratis and the Yemeni separatists is that they collaborate with terrorists. The Hadi government, in turn, refers to the Emirati-backed separatist forces as "outlawed groups".

Government and southern forces have clashed periodically before but the current conflict is the most serious since the war with the Houthis began four years ago. Similarly, there have been occasional reports of Emirati forces taking part – but on a small scale. 

The latest confrontation started after two separate attacks killed dozens in Aden on 1 August. A missile strike by the Houthis hit a military ceremony involving southern forces, killing Abu al-Yamamah, a prominent separatist commander, along with many others. Earlier in the day a suicide attack attributed to Islamist militants hit a police station.

Separatists accused the government of failing to provide adequate protection and on 10 August, following several days of clashes, they took control of Aden. From there, they extended their influence to other parts of the south, making relatively easy gains until they met fierce resistance in Ataq, in Shabwa province.

Following a battle in and around Ataq on 22 August, separatist forces throughout Shabwa capitulated rapidly, forfeiting their earlier gains in the province. On Wednesday pro-government forces continued their advance, moving rapidly through the neighbouring Abyan province – apparently with little resistance – to reach Aden, and for a few hours they were claiming control of the city.

On Thursday, though, events took another dramatic turn – almost certainly as a result of the UAE's intervention. Following the Emirati airstrikes the Hadi government said it had withdrawn its forces from Aden to prevent the city from being destroyed. 

The separatists are also reported to have recovered some lost ground in Abyan province, re-capturing Zinjibar – again, apparently as a result of Emirati airstrikes.

One political conclusion to be drawn from this is that the UAE is not prepared to abandon the southern forces for the sake of its relations with Saudi Arabia. It has invested a lot in training and supporting them, partly to fight the Houthis and partly in the hope of bringing some stability to the south. 

However, the UAE also seems to be using them to put pressure on the Saudis for reform of Hadi's government.

The Hadi government was established in 2012, supposedly for a two-year "transitional" period, after popular protests led to the resignation of President Ali Abdullah Saleh but was later driven out of the capital, Sanaa, by the Houthis. Since then, Hadi has spent most of his time in Saudi Arabia.

Restoring Hadi's "legitimate" government was the official reason for the Saudi-led intervention that began in 2015 – and in theory it still is. Under Resolution 2216, approved by the UN Security Council in 2015, all other countries are also required to support Hadi's legitimacy.

While this was an understandable position at the time, it has become increasingly problematic. Whatever legitimacy the government once had has now mostly evaporated. Hadi himself is an uninspiring leader with little popular support inside Yemen and with a notorious military figure – Ali Muhsin al-Ahmar – as his vice-president. The government itself is hopelessly ineffectual.

By implication, Resolution 2216 also instructs other countries to have no truck with the southern separatists: it calls on all UN members to refrain from any action that undermines Yemen's unity and territorial integrity. 

One of the separatists' complaints – and a factor behind the current fighting in the south – is that the southerners are not adequately represented in the UN's plan for ending the wider war in Yemen. The aim of the plan is to establish a transitional government and a national military security council. This would be negotiated between the Houthis and the Hadi government – the southerners would not be brought in until a later stage, as part of "inclusive political talks."

The separatists see their current military action in the south as a step towards eventual independence but – more immediately – as a way of acquiring leverage in discussions about the post-war shape of Yemen.

To that end, at a meeting with UN special envoy Martin Griffiths on 6 August, the Southern Transitional Council's leader, Aidarus al-Zubaidi, asked for the southerners to be represented in the peace process "as a principal and independent party". 

That's more than the UN is willing to swallow at the moment, though in a statement on Thursday the Security Council did say it supports "an inclusive dialogue to resolve differences and address the legitimate concerns of all Yemenis, including those in the south".