Separatists sent reinforcements to Yemen's southern city of Aden on Monday as fighting continued with government units, security and humanitarian sources said, a day after the secessionists seized government headquarters.

The separatist takeover on Sunday in the interim capital threw war-torn Yemen into further chaos, sparking warnings of a "coup" from the embattled government.

It further complicates years of conflict, in which the Saudi-backed government has been battling Iran-backed rebels controlling the capital Sanaa and much of the north of the country.

On Monday, the separatists, who have long demanded independence for the south, dispatched additional forces from the central province of Marib and southern province of Abyan, security sources said.

The forces from Abyan progressed towards Aden after clashes with government forces on the way, they said.

The International Committee of the Red Cross reported fighting overnight in Aden.

"All night shooting in Aden #Yemen, including heavy weapons," Alexandre Faite, the head of the ICRC delegation in the country based in Sanaa, said on Twitter.

"Those in southern part of city, including (ICRC staff) still unable to get out."

On Sunday, security sources said pro-separatist units trained and backed by the United Arab Emirates had taken over the government headquarters in Aden after clashes.

The fighting killed 15 people including three civilians, after separatist protesters were prevented from entering the city for a rally to demand the government's ouster from Aden.

The separatists accused the prime minister of ordering his troops to open fire at the protesters.

Sunday's rally was called by the Southern Transitional Council, an autonomous body not recognised by the government and aimed at overseeing self-governance in southern provinces.

Former Aden governor Aidarous al-Zoubeidi formed the council in May last year after the President Abedrabbo Mansour Hadi fired him.

South Yemen was independent -- with former British colony Aden as its capital -- from its formation in 1967 until 1990, when it was unified with North Yemen.

More than 9,200 people have been killed in Yemen since a Saudi-led military coalition intervened to support Hadi's government in its fight against the rebels in 2015.

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