The Saudi-led air coalition targeting Shia Houthi rebels in Yemen has mistakenly hit a pro-government position, killing at least 20 troops, security officials and witnesses say.
At least another 20 people are thought to have been injured.
The strike occurred between the southern Taiz and Lahj provinces, which have seen fierce clashes recently.
"They thought the Houthis were still there," a pro-government security official told The Associated Press.
The Saudi-led coalition is backing forces in Yemen loyal to the exiled President Abdrabbuh Mansour Hadi.
Pro-Hadi forces retook Lahj and four other provinces in mid-July as part of a southern offensive.
But fighting has raged for months in Taiz, which the rebels have held since last year and is seen as a gateway to the capital Sanaa.
A separate coalition air strike on Saturday killed 13 Houthi rebels in the desert province of Jawf, AP reported, and three pro-government fighters were killed in rebel rocket shelling in Marib province east of Sanaa.
Shia fighters, known as Houthis, stand around a crater made by a Saudi-led coalition airstrike in Sanaa, Yemen, Friday, October 16, 2015.
Hundreds of Sudanese troops reportedly arrived in the southern port city of Aden on Saturday, the first batch of an expected 10,000 reinforcements for the Saudi-led coalition.
Their mission is to secure Aden, which has seen a surge of assassinations of pro-government military leaders in recent weeks.
On Friday, gunmen on a motorcycle killed an Saudi officer in a drive-by shooting.
Around 4,500 civilians have been killed in the conflict since March, according to the UN.
As well as those killed, nearly 1.5 million people have been displaced by the conflict. The UN estimates that 13 million people face food shortages and 80% of the population need some form of aid.