Key players in Yemen's slide into chaos
The war in Yemen pitches forces loyal to an internationally recognised government backed up by Saudi Arabia against rebels supported by Iran who have controlled the capital for over three years.
More than 8,750 people have been killed as armed groups have sought to exploit a power vacuum left behind after protests in 2011 forced out the now late president Ali Abdullah Saleh.
The Huthi rebels
The Huthi militiamen, also known as Ansarullah (Supporters of God), have long complained of marginalisation.
They hail from the Zaidi sect, an offshoot of Shiite Islam that accounts for about one third of Yemen's population.
Their strongholds lie in northern provinces bordering Sunni powerhouse Saudi Arabia, which leads the Arab coalition against them.
Badreddin al-Huthi, who formed the "Faithful Youth" political movement in 1992 to fight discrimination, is regarded as their spiritual leader.
They fought six wars with the central government between 2004 and 2010 that killed thousands. Badreddin's 38-year-old son Abdulmalik now leads Ansarullah.
The Huthis seized Sanaa on September 21, 2014 with the help of former president Saleh and army units loyal to him after months of clashes.
In March 2015, they advanced on second city Aden, where internationally backed President Abedrabbo Mansour Hadi took refuge after escaping house arrest in the capital.
But the Saudi-led coalition helped pro-Hadi forces push the rebels out of Aden in July that year, as well as four other provinces.
The Huthis are accused of receiving support from Shiite Iran and Lebanon's Hezbollah. Tensions ratcheted up between the Saudis and Tehran after the Huthis fired a ballistic missile at Riyadh in November 2017.
Once foes, the Huthis and former leader Saleh formed an uneasy alliance that lasted some three years but it cracked up last week as violent clashes broke out between the two camps in Sanaa.
The Huthis accused Saleh of "great treason" after he made overtures to the Saudi-led coalition.
Saleh, who dominated Yemen for three decades, was shot dead by Huthi fighters on December 4 as he tried to flee Sanaa. Despite his demise, the strongman's General People's Congress party remains a power with which to be reckoned.
President and foreign allies
Yemen's military was severely weakened due to mass defections of elite troops joining pro-Saleh forces and showed little or no resistance as the rebels seized Sanaa and expanded across the country.
But in March 2015, the Saudi-led coalition launched a military campaign in support of leader Hadi, deploying troops and beginning a bombing campaign that helped the loyalists regain some ground.
The United Arab Emirates also plays a key role in the coalition, which includes Bahrain, Egypt, Qatar and Sudan.
Pro-government forces last January began a major assault to recapture areas on the Red Sea coast and press on to Sanaa. However, the offensive has stalled and the frontlines remain relatively static.
Hadi loyalists have been boosted by the Popular Resistance alliance of southern separatists and tribesmen who took up arms after the rebels advanced on their regions.
The separatists have long called for the secession of the formerly independent South Yemen and their support for Hadi is not unconditional.
Al-Qaeda in the Arabian Peninsula (AQAP) has been regarded by the United States as the extremist network's deadliest branch.
It was formed in 2009 when Al-Qaeda in Yemen -- behind the 2000 bombing of the USS Cole in Aden harbour that killed 17 American sailors -- merged with its Saudi counterpart.
AQAP has attacked security forces and the Huthis, and been targeted by scores of US drone strikes.
The United States has intensified its air attacks on AQAP since President Donald Trump took office in January.
The group has abducted foreigners and claimed responsibility for the deadly January 7, 2015 attack in Paris on French satirical magazine Charlie Hebdo, targeted for its cartoons of the Prophet Mohammed.
Islamic State group
The Islamic State group has claimed deadly attacks against Huthis but has also targeted loyalist militia and government officials.
The group first surfaced in Yemen in March 2015, claiming multiple suicide bombings that targeted two mosques in Sanaa attended by Huthis, killing 142 people and wounding more than 350.
It has since expanded its operations in the south, launching more deadly suicide bombings.
© 2017 AFP