The extremists were travelling towards the city of Aden, the government's temporary base, when loyalists manning a checkpoint in neighboring Abyan province stopped them, sparking a gunfight, the sources said.

The three suspects were coming from the southeastern city of Mukalla, which Al-Qaeda in the Arabian Peninsula (AQAP) has controlled since April.

Yemen, home to what the United States considers Al-Qaeda's most dangerous affiliate, has been convulsed by unrest since Iran-backed Huthi rebels seized Sanaa in September 2014.

AQAP has exploited the turmoil to tighten its grip on parts of southeast Yemen, including Mukalla, imposing a strict form of Islamic law.

Islamist militants, including AQAP and ISIS, have gained ground in and around Aden.

Al-Qaeda fighters have also seized the town of Azzan in Shabwa province east of Abyan, local officials and tribal chiefs said on Monday.

The authorities have struggled to exert control over several southern regions that loyalists have recaptured from the Huthis and their allies since July.

There has been a string of bombings and assassinations, many claimed by ISIS, targeting security forces and government officials.