Saudi-led coalition jets targeting Houthi militiamen in Yemen killed 30 fighters loyal to Yemeni President Abd-Rabbu Mansour Hadi when they mistakenly bombed a military camp in the province of Taiz on Saturday, local officials said.
The air strike, in which another 40 fighters were wounded, was the latest in a series of "friendly fire" incidents that have plagued the Saudi-led campaign against the Iranian-allied Houthis since it began in March.
Officials from the Saudi coalition and the Yemeni government could not immediately be reached for comment.
Saturday's strikes hit a mountainous area close to the southwestern city of Taiz where fighting is intense between local fighters and troops loyal to veteran former president Ali Abdullah Saleh, who has sided with the Houthis, government officials told Reuters.
"The coalition raid hit the wrong target," a government official said.
Last month 131 people were killed when missiles fired from coalition planes hit a wedding party in the Red Sea village of al-Wahijah, near the ancient port of al-Mokha.
At least 5400 people have been killed in the six-month civil war in the poorest country on the Arabian Peninsula, and the United Nations says the humanitarian situation, exacerbated by a Saudi blockade of the country's ports, is "critical".
Saudi Arabia has been leading a military intervention since March to try to restore Hadi's government, now based in a hotel in Aden, and fend off what it sees as creeping Iranian influence.
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 4500 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 percent of the population need some form of aid.