Yemen's southern coastal city of Aden has been gripped by days of fighting after armed separatist forces - backed by the United Arab Emirates (UAE) - moved against the internationally recognised government.

Fighters from the Southern Resistance Forces (SRF), the armed wing of the Southern Transitional Council (STC) - a political movement demanding secession for southern Yemen - clashed with the Yemeni army and were able to wrest control of a key military base in Aden's Khormaksar district and capture scores of soldiers.

The STC is said to have precipitated the crisis by handing President Abd-Rabbu Mansour Hadi's government an ultimatum last week to either dismiss Prime Minister Ahmed bin Dhagr and his cabinet or face an overthrow.

The group accused Hadi's government of "rampant corruption" resulting in a "deteriorating economic, security and social situation never before witnessed in the history of the south".

Hadi's government refused the ultimatum and subsequently banned public gatherings ahead of Sunday's deadline.

Residents told Al Jazeera that when the Yemeni army was deployed early on Sunday to prevent separatists from entering the city, it triggered the fierce confrontation.

At least 12 people were killed in the clashes and more than 130 wounded before Hadi's government called an immediate ceasefire.

Fighting didn't subside, however, and residents told Al Jazeera on Monday that gunfire continues throughout the city.

Who are the separatists?

Separatists have long campaigned for the secession of southern Yemen, which was an independent country before 1990.

Less than four years after merging with the north, the south tried to split away in 1994 citing economic and political marginalisation, but it was crushed after a short-lived but bloody civil war.

Since 2007, southern groups have been rallying for greater autonomy, and those calls intensified after the 2011 Arab spring uprising and the outbreak of the 2015 Yemeni civil war.

The STC, formed in 2017 by Aidarous al-Zubaidi, a 51-year-old militia leader, says Hadi's government, which is allied to Islah - a group ideologically linked to the Muslim Brotherhood movement - should sack his prime minister and cabinet and exclude Islah-aligned leaders from the political leadership.

Who's fighting who?

The UAE, a major patron of the STC, has invested millions of dollars in the group and pro-autonomy Salafis in a bid to secure its interests in the region.

With a sizeable budget, the STC has been able to rally a number of tribes to its cause and develop a large following in the coastal cities of Aden and Mukalla, as well as the provinces of Dhale and Lahij.

Despite hailing from the south himself, Hadi's support has been restricted to the provinces of Abyan and Shabwa following bloody events that rocked the south in January 1986.

Shallal Shayae, Aden's security chief and a senior commander of the SRF, has had a long and personal grudge against the president after his father was assassinated by forces close to Hadi.

Hadi was aligned with then President Ali Nasser Mohamed, whose bodyguards carried out the killing. The duo would flee the south to north after losing the war, which claimed close to 10,000 lives and divided the south along ideological and tribal lines.

However, most of Hadi's power comes from Saudi Arabia, where he has been based since 2016. The Saudis formed an Arab coalition and launched attacks against Houthi rebel forces in 2015 and Riyadh supports Hadi's forces with military assistance and financial largesse.

What's happening right now?

The STC has blamed Prime Minister Ahmed bin Daghr for the violence and urged Hadi to sack him and his cabinet.

"The STC holds the bin Daghr government fully responsible after it violated the Arab coalition's call for calm and used weapons to prevent demonstrators from reaching the parades square," it said in a statement.

The STC has said it is willing to de-escalate the situation, but Zaid al-Jamal, al-Zubaidi's secretary, told Al Jazeera the "uprising" would continue until Hadi's government was "toppled".

International aid organisation Oxfam says the violence has forced it to close down its offices in Aden, while residents have complained of schools and government institutions being forced to shutter.

What is the UAE's role?

The UAE is believed to be sponsoring southern Yemen's secession to advance its interests in the region.

The Gulf emirate entered Yemen's war in March 2015 as part of the Saudi-led military coalition after Houthi rebels - traditionally based in the northwest of the country - seized Sanaa, the capital, and claimed they were the legitimate government.

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Despite having a relatively small army, the UAE sent a significant number of ground forces to Yemen. In contrast, Saudi Arabia was cautious to deploy troops; the Saudi National Guard and Royal Saudi Land Forces (RSLF) play minimal roles in the conflict.

The UAE's interest relates to the security of the Bab el-Mandab strait, one of the world's busiest oil and gas shipping lanes.

Protecting the flow of oil and gas shipments in the Red Sea and Egypt's Suez Canal is vital for UAE's ability to trade with Europe and North America.

Divisions between UAE-Saudi over Yemen?

Nearly three years on, and with fighting showing no signs of abating, Saudi Arabia has said it "wants out" of the war, which is believed to cost the kingdom an estimated $66m a day.

Conversely, the Emiratis have become more involved in the conflict, indicating a divide in the two countries' agendas.

The UAE has been financing and training armed groups that only answer to it, setting up prisons, and creating a security establishment parallel to Hadi's government, according to Human Rights Watch.

The Middle East Eye news website, quoting sources, reported that Hadi was incensed with the UAE, accusing Abu Dhabi Crown Prince Mohammed bin Zayed of acting as an occupying force as opposed to a liberation one.

The weakening of Hadi's government has gone hand-in-hand with the UAE's growing power. According to Maysa Shuja al-Deen, a non-resident fellow at the Sanaa Center for Strategic Studies, the Emiratis seem disillusioned with Saudi Arabia's plan for the country.

"The Saudis believe any talk of secession will de-legitimise the war effort, which they have repeatedly claimed is about restoring the government of President Hadi. Meanwhile, the Emiratis don't want to see any party close to Hadi and Islah anywhere near power.

"The coalition is divided and no longer knows what they want," she said.