The BBC has found evidence in Yemen that troops from a Saudi-led coalition force and al-Qaeda militants are both fighting Houthi rebels in a key battle.
On a visit to the frontline near the city of Taiz, a documentary maker filmed jihadists as well as UAE-supported pro-government militiamen.
The coalition of 10 mostly Sunni Arab states is backing Yemen's government in its war against the Shia rebels.
But it denies co-operating with Sunni extremists also opposed to the Houthis.
The coalition's member states consider al-Qaeda a terrorist organisation, and the jihadist network's local affiliates have attacked coalition forces and Yemeni government personnel.
At least 6,000 people have been killed in Yemen since March 2015, when the coalition launched a military campaign to defeat the Houthis and allied army units loyal to former President Ali Abdullah Saleh and restore the government.
'Spreading the message'
Pro-government forces have been battling the rebels for control of Taiz, about 205km (123 miles) south of the rebel-held capital Sanaa, for months.
The Houthis control all routes into and out of the city, and are besieging a Sunni Islamist-dominated alliance of local forces holding the city centre, while coalition-led forces are attacking the rebels on several fronts to the south and west.
Taiz has suffered huge destruction as a result, and the UN says some 200,000 civilians are trapped inside the city without critical medical supplies or food.
During a visit to the frontline outside Taiz late last year, documentary maker Safa AlAhmad spoke to pro-government militiamen attacking Houthi fighters on a key hilltop with the support of troops from the United Arab Emirates (UAE), who were providing tactical advice.
While there, Ms AlAhmad was warned by one group participating in the battle not to film them.
She was told they were members of Ansar al-Sharia, an affiliate of al-Qaeda in the Arabian Peninsula (AQAP), and that they were angered by the presence of a woman.
Ms AlAhmad said it appeared that AQAP - which has exploited the chaos and seized parts of southern Yemen, including the port city of Mukalla - had sent fighters to Taiz to increase the group's influence and spread its message.
Several reports of coalition forces and AQAP militants battling the Houthis in the same areas in southern Yemen have emerged over the past 11 months, despite the jihadists' long-standing violent opposition to governments of coalition-member states, who are allied with the US.
Some have alleged that the Yemeni government is avoiding direct confrontation with AQAP, which in turn has avoided attacks on government targets.
The Houthis have claimed that Saudi Arabia and its Sunni allies are more concerned with checking the influence of Shia power Iran, which has denied providing military support to the rebels, than combating al-Qaeda.