Yemen explained: Is the fight for Aden about to become the new international war by proxy?

Rebels in Yemen have seized an air base outside the critical southern port city of Aden – a development which spells disaster for those loyal to President Abd-Rabbu Mansour Hadi.

But as the country descends into further chaos, the civil war is increasingly drawing in parties from across the entire region, and the battles lines, allegiances and even which countries are involved is becoming increasingly hard to understand.

Who are the rebels?

Well organised and powerful, the rebels are a group of Shia Muslims from the Zaidi sect known as the Houthis.

Last month they drove President Hadi out of the Yemeni capital Sana’a, and while they now have a strong presence in the country’s north they are week – and heavily opposed – by many Sunni tribes across the rest of the country.

Who is in government?

President Hadi came to power in elections after anti-government protests saw his now Houthi-backed predecessor President Ali Abdullah Saleh step down in 2011.

That transition was overseen by a host of neighbouring countries, and his rule is backed by the UN.

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The Sunni government – and neighbouring Sunni countries – reject the Houthi rebels’ move into Sana’a in September 2014 as a military coup.

What other groups are involved within Yemen?

Al-Qaeda is well-embedded in Yemen’s south and south-east, and during the civil war has staged repeated deadly attacks under the guise of its local affiliate al-Qaeda in the Arabian Peninsula (AQAP).

AQAP’s involvement cannot be described as sectarian, however – the militant group opposes both President Hadi’s government and the Houthi rebels.

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And the picture has been confused further by the rise in late 2014 of a Yemeni branch of Isis. Though still small, it wishes to oppose its extreme version of Sunni Islam and opposes the government, the Houthis – and AQAP.

Isis is accused of carrying out a mass suicide bombing on Shia mosques in Sana'a last week that killed at least 137 people.

And other countries?

The greatest danger with the situation in Yemen, Jon Altman of US-based Centre for Strategic & International Studies told BBC News, is that it becomes “a proxy war between the Gulf Co-operation Council states and Iran”.

That league of Arab states includes Qatar, Oman and Saudi Arabia – among others – backs President Hadi’s government. The Saudi leadership, in particular, does not want to see a Shia Muslim country established on its southern border.

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On Wednesday afternoon Saudi military officials confirmed that heavy weapons including artillery had been moved to bolster areas near its Yemeni border – but insisted “this is only to defend the country”.

Iran, meanwhile, is reported to have begun supporting the Houthi rebels militarily. While rebels officially deny this, there are unconfirmed reports of trained Iranian pilots flying Yemeni planes and senior Houthi figures sighted in Iran.

How likely is an international conflict?

On Tuesday, President Hadi asked the UN Security Council to authorise a military intervention by the Gulf Cooperation Council “to protect Yemen and to deter the Houthi aggression”.

The US and UK have evacuated their diplomatic staff from the country – and even a small US military base was recently disbanded after AQAP took over a town nearby.

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But at the moment it remains – internationally at least – a war of words. Saudi Arabia said today that “if the Houthi coup does not end peacefully, we will take the necessary measures for this crisis to protect the region”, still leaving open the possibility of a de-escalation.

Security analyst Aimen Deen from the think tank Five Dimensions said: “The pressing question is whether the Royal Saudi Air Force will intervene to prevent Aden from falling to the Houthis. All indications are that the Saudis are preparing militarily to answer this question, but the political decision is not yet taken.”

So what’s the latest on the ground?

Yemeni officials have confirmed that forces “allied with the Shia rebels” have taken over the Aden airport.

President Hadi has fled his palace home in the city for an undisclosed location, suggesting the government expects the Houthi advance to continue. Officials said he was still coordinating his forces’ response.

The takeover of Aden, the country's economic hub, would mark the collapse of what is left of President Hadi's grip on power, according to the Associated Press.

Also on Wednesday, senior government figures including defence minister Maj Gen Mahmoud al-Subaihi and his top aide were taken by rebels and transferred to Sana’a.

What might happen next?

It seems likely the Houthi rebels will focus on attempts to return President Hadi to house arrest, though reports are emerging that he may already have fled the country.

Rebels already control Yemen’s state TV station, which has issued extraordinarily a public appeal for President Hadi’s capture and offered a bounty of nearly $100,000 (£67,000).

If the Houthis continue to establish a tighter grip on power, the Arab countries currently backing President Hadi will face a tough decision. As opposed as they are to the idea of the government falling, an all-out war in Yemen would have no certain outcome, no clear exit strategy – and no simple solution.