Yemen is already split between the Iran-backed Houthis who control much of the north and a fractious patchwork of anti-rebel groups in the internationally recognised government.
The separatists of the Southern Transitional Council are part of that anti-Houthi government, but their advances have raised fears that the group might secede in an effort to revive the once independent South Yemen.
WHAT IS THE STC?
Headed by Aidaros Alzubidi, the STC is a coalition of groups that want to bring back South Yemen, which existed from 1967 until its unification in 1990 with North Yemen.
They now control almost all of South Yemen's former territory.
The STC has gained influence during Yemen's civil war, which has pitted the Houthis against forces backed by a Saudi-led military coalition that includes the United Arab Emirates.
Close to Abu Dhabi, the separatists are part of the Presidential Leadership Council (PLC), the eight-member body that heads the internationally recognised government.
Alzubidi is the PLC's vice-president while Saudi-backed Rashad al-Alimi, who has heavily criticised the separatists' advance, is its president.
The STC already controlled swathes of Yemen's south coast, including al-Mukalla, the capital of the country's largest province Hadramawt.
Last week, its forces swept inland, seizing the key city of Seiyun as well as oil fields in the mostly desert area bordering Saudi Arabia.
In recent days, some local leaders in neighbouring Mahra province, which borders Oman and is a key smuggling route, also joined their alliance, the STC told AFP.
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