Al-Qaeda in the Arabian Peninsula (AQAP), long active in Yemen, and the emerging Islamic State group appear to be vying for influence in the main port city, to which President Abedrabbo Mansour Hadi returned from exile in November.
Fighting between Iran-allied rebels and Saudi-backed loyalist forces has plunged the impoverished nation into chaos, which both jihadist groups have exploited to make sweeping gains, particularly in southern regions.
The Saudi-led coalition battling the Shiite Huthi rebels since March has not targeted the jihadists.
In recent days, armed and sometimes masked jihadists have been massively deployed in Aden's central Tawahi district, where governor Jaafar Saad was killed Sunday in an attack claimed by IS.
"We live in fear. We don't know where those gunmen roaming on our streets came from," said 35-year-old resident, Ahmed. "Tawahi has turned into a ghost town."
The gunmen have set up roadblocks in Tawahi and are spotted in other Aden districts such as Mualla, Dar Saad, Sheikh Othman, and Mansura, according to residents.
Their typical targets are government officials -- policemen, soldiers, or even judges.
On Saturday, assailants shot dead the presiding judge of a terrorism court, Mohsen Mohamed Alwan, and four of his bodyguards.
Two police colonels were gunned down in other attacks at the weekend.
Civilians have not been spared.
'HEADING TOWARDS CATASTROPHE'
On Monday gunmen killed a resident, Abdulaziz Ahmed, in Tawahi over accusations he was practising witchcraft.
"As night descends, these men move around in their vehicles spreading terror," said Saleh Ahmed, a resident of Mansura.
They are members of "Daesh or Ansar al-Sharia," he said, using the Arabic acronym for IS and another name for AQAP.
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Seen by the United States as Al-Qaeda's deadliest branch, AQAP had been alone on the jihadist scene in Aden until IS surfaced this year claiming deadly attacks, including one that targeted the government's temporary headquarters in October wounding several ministers.
"Aden is heading towards catastrophe if the situation persists and the government does nothing to quickly restore security," said Ahmed.
Last month, radical Islamist gunmen entered a university faculty in Aden, forcing students to leave the campus and locking the main gate, witnesses said.
They closed the faculty after threatening to use force against students if they did not observe segregation of the sexes on campus.
The jihadists are occupying several government buildings in Aden and have even seized newsstands to distribute their leaflets, according to witnesses.
Yemeni sources and Arab experts point accusing fingers at former president Ali Abdullah Saleh, an ally of the rebels who ruled Yemen for three decades and still wields influence in the deeply tribal country.
JOINING JIHADISTS FOR MONEY
A military analyst in Aden, who asked to remain anonymous, said that Saleh, whose supporters are fighting alongside the Huthis, was using jihadists as a "tool" against the internationally recognised government.
But security sources also say AQAP and IS are recruiting fighters from the Popular Resistance, an alliance of loyalist militiamen and troops who have become frustrated by unfulfilled promises to merge them with the security forces.
"Out of 59,000 Popular Resistance candidates who had applied, only 1,500 fighters have been integrated into the police and army in Aden, Daleh, Abyan, and Lahj," said a senior security official, referring to four of five southern provinces recaptured by Hadi loyalists since July.
Due to a lack of funds, "the initial list of candidates was reduced the first time to 20,000, and then again to 15,000," said the official who requested anonymity.
Hadi on Monday appointed the leader of the Popular Resistance in Daleh, Aidarus al-Zubaidi, as governor of Aden and his deputy, General Shallal Ali Shayae as its police chief.
As the announcements were made, authorities deployed some 500 fighters in Aden, mainly around Tawahi and neighbouring districts, security sources said.
The two new "strongmen of Aden", who returned to the city this weekend from key coalition pillars Saudi Arabia and the United Arab Emirates, are expected to deploy "up to 5,000 members of the Popular Resistance" across the port city, a security official said.
"These men, trained at military camps run by the coalition in Eritrea and Yemen, will be charged with securing Aden," another source said.
AQAP fighters on Wednesday drove pro-government forces out of a town in southern Yemen that could potentially open up a supply route between Aden and their stronghold of Mukalla, in Hadramawt.