Yemen's Prime Minister Ahmed Obaid Bin Daghar described the separatists' move as a coup while others pointed to the UAE's role in unleashing forces trained and armed by the Gulf Arab state to attack government offices, forces and allies.
At least 15 soldiers were killed in a suicide auto bombing attack on Tuesday in Yemen's southern province of Shabwa, a security source said.
The fighting further intensified in 2015, when clashes broke out between Yemeni President Abdrabbuh Mansur Hadi's government forces and Saleh loyalist forces.
Aden is meant to be the "interim" capital of the pro-Saudi Hadi government, but is also the historical capital of South Yemen.
Hundreds of soldiers and senior officers allied with Yemen's internationally-backed President Abdu-Rabbu Mansour Hadi were arrested by the STC forces.
Almost three years on, the coalition has failed to achieve its stated aim of restoring the "legitimate" government of Hadi.
At the same time, a key member of that coalition, the United Arab Emirates, has closely backed the STC with an eye toward maintaining crucial oil shipping routes in Yemen's south.
Yemen fell into civil war in 2014 when Houthi rebels overran much of the country, including capital Sanaa, forcing Yemen's Saudi-backed government to set up an interim capital in Aden.
He urged a Saudi-led military coalition backing Hadi to intervene in its defence.
A auto bomb detonated killing at least 11 people at a security forces-run checkpoint in southeastern Yemen.
The separatists, known as the Southern Transition Council, are seeking the revival of the independent state of South Yemen, which existed before Yemen was unified in 1990.
Hadi's government has accused the separatists of attempting a coup in Aden, opening yet another front in the country's devastating conflict.
Violence first erupted on Sunday when a deadline issued by the separatists for the government to resign expired.
United Nations envoy to Yemen Ismail Ould Cheikh Ahmed urged all parties to return to "calm and dialogue".
The Red Cross says the fighting has left 38 people dead since Sunday.
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