Yemen's prime minister has accused southern separatist forces backed by the United Arab Emirates (UAE) of staging a "coup" after they seized several government offices during deadly clashes in the port city of Aden.

"In Aden, legitimacy is being overturned," Prime Minister Ahmed bin Dagher said in a statement on Sunday.

"What is happening is very dangerous and affects the security, stability and unity of Yemen... This wrongdoing is no different than the crimes committed by the Houthis in Sanaa," he added.

Clashes erupted in Aden early on Sunday after the army of President Abd-Rabbu Mansour Hadi, who is supported by Saudi Arabia, tried to prevent separatists, backed by the UAE, from entering the city.

At least 10 people were killed and 30 wounded in the fighting, hospital sources were quoted as saying. The main airport was also reported closed.

The Southern Transitional Council (STC) - a movement demanding secession for southern Yemen - had given Hadi's government a seven-day ultimatum last week to either dismiss his prime minister and cabinet, or face an overthrow.

Hadi's government refused to be coerced and subsequently banned public gatherings ahead of Sunday's deadline.

Murad Abdu, a youth activist in southern Yemen, told Al Jazeera the deployment of Hadi's presidential guard sparked the fierce confrontation with the STC's armed forces.

Heavy gunfire reverberated throughout much of the city with pitched street battles in Aden's Khormaksar, al-Mansoura, and Dar Sad districts, Abdu said.

The streets were empty of traffic with schools, government offices, and most shops closed.

Hani bin Braik, vice president of the STC, blamed Hadi's government for the fighting in a Twitter post.

"They forced us to put on our military uniforms, although we told them we were non-violent. But we were ready," he said.

Responding the announcement, Zaid al-Jamal, secretary of STC leader Aidarous al-Zubaidi, vowed the "uprising" would continue until Hadi's government was "toppled".

"We have announced a new programme of popular uprising that will start tomorrow. People have already started flooding into al-Orouth Square and will not leave until the government is overthrown."

Saudi TV channel Al-Hadath reportedthat Hadi called for an immediate ceasefire and for all sides to withdraw fighters from the city.

Calls for a ceasefire came "too late" with the secessionists firmly in a position of "control", another member of the STC said.

"I expect [Saudi Arabia] to intervene and put pressure on [Hadi's] government to resign," the source said on condition of anonymity because he was not authorised to talk to Al Jazeera.

"Hadi only called for a truce to save what remains of his soldiers," he said, adding fighting had reached a "decisive" moment.

The source also said there were casualties on both sides with at least 24 people confirmed dead, and heavy gunfire still echoing throughout the city late into the evening.

UAE involvement

The latest developments in Aden underscore the rising tensions between Hadi's government and the southern separatists.

UAE entered Yemen's war in March 2015 as part of a Saudi-led coalition seeking to remove Houthi rebels. Traditionally based in Yemen's northwest, the Houthis overran much of the country, including the capital Sanaa, in 2014.

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The UAE has been financing and training armed groups in the south who answer to the STC's leader Zubaidi, 50, who emerged from relative obscurity in late 2015 after helping purge the Houthis from Aden.

Zubaidi was initially rewarded and made governor of Aden by Hadi, but he soon fell out of favour after reports emerged he was receiving patronage from the UAE to campaign for secession.

The weakening of Hadi's government has gone hand-in-hand with the UAE's growing influence in southern Yemen.

The Gulf nation has financed a network of militias that only answer to it, set up prisons, and created a security establishment parallel to Hadi's government, according to Human Rights Watch.

The Saudi-led coalition has so far failed to achieve its stated aims as Houthi rebels continue to hold Sanaa and much of the north.

Yemen's war has taken a huge toll with more than 10,000 people killed, millions at risk of famine, and a massive cholera outbreak.

Saleh Al Batati contributed to this report