One of two conjoined twins from war-torn Yemen has died after a "complicated" 15-hour operation to separate them, Saudi state media reported Monday.

The 19-month-old baby boys, Yussef and Yassin, were "conjoined in several organs", and some 24 doctors were involved in the operation, the official Saudi Press Agency (SPA) said. 

On Monday morning the SPA hailed the procedure as a success, but a follow-up report Monday night revealed one of the twins had died "as a result of a severe drop in blood circulation and heart failure, despite the full medical care provided for him".

Doctors "faced great difficulties and challenges during the separation process, which made the deceased's condition critical after the operation", the report said, adding that the surviving twin was in a stable condition.

It did not specify which twin lived and which died. 

Yemen has been wracked by a brutal seven-year conflict pitting the Saudi-backed government against Iran-backed Huthi rebels, whose seizure of the capital Sanaa in 2014 prompted a Saudi-led military coalition to intervene. 

More than 150,000 people have died in the violence and the country's health system has been devastated, in what the United Nations describes as the world's worst humanitarian crisis. 

Saudi Arabia's state-run King Salman Humanitarian Aid and Relief Center (KSRelief) regularly touts its humanitarian assistance to Yemen as evidence of Riyadh's commitment to alleviating the suffering there. 

The centre's doctors carried out the "four-phase surgery" separating Yussef and Yassin, describing it as "among the most complicated" they had performed, the SPA said Monday. 

Last July Saudi doctors separated a Yemeni baby from her parasitic twin, saying at the time it was their 50th successful operation on conjoined twins. 

In December a separate set of Yemeni twins were separated by doctors in Jordan's capital Amman before being flown back to Sanaa, according to the UN children's agency. 

Last week Saudi King Salman ordered that yet another set of Yemeni conjoined twins, girls named Mawaddah and Rahmah, be transferred to Riyadh "to conduct medical examinations and check on the possibility" of separation, the SPA reported. 

The king "attaches big importance to the Saudi programme for the Siamese twins", the SPA report said.