Seven civilians have been shot dead by Kenyan troops advancing towards the al-Shabab stronghold of Kismayo, a Somali army spokesman has told the BBC.
"It was a deliberate killing," said Adan Mohamed Hirsi.
The Kenyans are part of the pro-government African Union mission in Somalia, which says it is investigating the incident.
The alleged shooting happened about 50km (30 miles) from Kismayo, the largest city still in militant hands.
Kenyan troops intervened in Somalia a year ago after a spate of cross-border attacks blamed on the al-Qaeda linked al-Shabab group.
Mr Hirsi condemned the latest killings and asked the Somali government to take action.
"This incident is very hurtful," he told the BBC's Somali service, saying a group of young men were shot outside a shop in the village of Janay Abdalla.
Two civilians were seriously wounded, he said.
It happened after Kenyan and Somali forces had earlier been ambushed by al-Shabab fighters.
Col Ali Aden, a spokesman for the African Union mission, known as Amisom, confirmed to the BBC that Kenyan and Somali troops were operating together in the area.
He said that appropriate action would be taken after the investigation was completed.
BBC East Africa correspondent Gabriel Gatehouse says Kenyan forces have in the past been accused of causing civilian deaths in Kismayo while shelling al-Shabab targets inside the city from ships operating off the coast.
Some 10,000 people have fled Kismayo in the past week, the United Nations refugee agency estimates.
Al-Shabab has been forced out of the capital, Mogadishu, and several other towns over the past year but still controls much of the countryside in south and central Somalia.
However, it still stages frequent attacks.
On Saturday, gunmen shot dead a member of Somalia's new parliament in Mogadishu.
Mustafa Haji Maalim was gunned down after leaving a mosque in the southern Waberi district following evening prayers, witnesses said.
The dead lawmaker was the father-in-law of former President Sharif Sheikh Ahmed, and is the first parliamentarian to be targeted since a new 275-member assembly was selected in August.
No-one has so far claimed the attack, though al-Shabab has previously vowed to kill government officials.
On Thursday, a double suicide attack in Mogadishu targeting a restaurant recently opened by Somalis from the diaspora killed 18 people.
Since the overthrow of President Siad Barre in 1991, Somalia has seen clan-based warlords, Islamist militants and its neighbours all battling for control.