Torn between a north controlled by Shiite rebels and a south dominated by the embattled president's allies, Yemen is mired in a proxy war between Iran and Saudi Arabia, analysts say.
The struggle threatens to push the impoverished Arabian Peninsula state further towards the abyss as the Sunni-Shiite sectarian divide grows wider.
Amid the chaos, Al-Qaeda militants are closing ranks with tribes of fellow Sunni Muslims to counter the expansion of the Shiite Huthi rebels.
In an unprecedented show of force, the Iran-backed Huthis, who overran the capital Sanaa unopposed in September, last week staged military exercises near the border with Sunni-heavyweight Saudi Arabia.
"In the face of Iran's Shiite expansionism, Sunni solidarity is building, led by Saudi Arabia," one diplomat who requested anonymity told AFP.
The kingdom to the north has always played a prominent role in Yemeni politics.
It hosted negotiations that helped end a year of deadly nationwide protests and led to a deal that eased out president Ali Abdullah Saleh in 2012.
And following a request by President Abedrabbo Mansour Hadi, Riyadh is again poised to broker talks aimed at ending Yemen's latest political impasse.
- Tehran warning -
When Hadi fled Huthi-imposed house arrest in Sanaa in February and resurfaced in Aden, Saudi Arabia was the first country to transfer its embassy there, in an open display of support for the beleaguered leader.
Tehran has openly denounced moves to make the southern port Yemen's temporary capital.
"Sanaa is the official and historical capital of Yemen and those in Aden who back disintegration or civil war are responsible for the consequences," Iran's deputy foreign minister Hossein Amir Abdollahian said last week.
He charged that Hadi, who once he reached Aden retracted a resignation tendered under duress in Sanaa, "would have done better to stay in Sanaa and keep to his resignation letter and not lead the country into crisis".
April Longley Alley, Yemen specialist at the International Crisis Group, argues that Tehran has received a "huge political return for very little investment".
Signup to our newsletter and follow us on Facebook and Twitter!
"As Saudi Arabia hardens its stance against the Huthis and seeks to roll back their gains, the Huthis are likely to seek closer ties with countries like Iran," she said.
This Tehran-Riyadh struggle is "both complicating and amplifying conflict in Yemen," she said.
Saudi Arabia fears that the Huthis, who swept down from the north and expanded the area they control to the shores of the Red Sea, now have their sights set on the strategic Bab al-Mandab strait.
This would give Iran proxy control over the key waterway to and from the Suez Canal, adding to its already strong presence in the Gulf's Strait of Hormuz through which much of the world's oil passes.
- Saudi switch -
Fearing the rise of extremist Sunni Islamists, especially the Muslim Brotherhood, Riyadh stopped backing Yemen's Al-Islah (Reform) party, thus giving the Huthis the upper hand, according to Lina Khatib, director of the Carnegie Middle East Centre.
"At one point Saudi Arabia favoured a Huthi domination (along the border) instead of the Muslim Brotherhood. Its role in Yemen then diminished, encouraging the Huthis to advance" south and seize large amounts of territory within weeks, she said.
The Huthi-run Saba news agency reported on Thursday that Iran will provide Yemen with crude oil for one year, and also establish a 165-megawatt power plant.
The agreement was made during a Huthi visit to the Islamic republic after an Iranian commercial flight landed in Sanaa on March 1 -- the first in many years and the fruit of an aviation accord with Tehran.
The Huthis insist that Iran does not meddle in Yemeni affairs.
"We've said it clearly: we reject all interference in our internal affairs, whether from Saudi Arabia, Iran or the United States," Huthi spokesman Mohammed Abdulsalam told AFP.
He accused Saudi Arabia of seeking to "create chaos" in Yemen.
"The risk of civil war or partition in Yemen" is increasingly raised in political circles, Khatib said.
Mohammad Sadeh Sadqian, head of the Centre for Arab Studies in Tehran, warned that the Saudi-Iran struggle for influence, already clear in Iraq and Syria, could have consequences elsewhere.
The struggle "could engulf other countries" if Riyadh and Tehran "do not sit around one table and discuss openly all points of dispute," he said.