At least two Somali civilians were unintentionally killed in US airstrikes carried out during the past 14 months, the US Africa Command (Africom) acknowledged on Monday.
Allegations of several other civilian deaths caused by US air attacks on Al-Shabaab targets during that period are still being investigated, Africom added.
Africa Command noted on Monday that it had previously acknowledged the unintentional killing of two Somali civilians in April 2018.
Africom investigations did not substantiate claims of additional civilian killings in a total of 19 US airstrikes since February 2019, Monday's statement said.
The newly substantiated finding of two civilian deaths pertains to what Africom describes as a US air attack on February 23, 2019, that also killed two Shabaab militants who were the intended targets.
Three civilians were also injured in the strike in the vicinity of Kunyo Burrow in southern Somalia, Africom said.
Those deaths and injuries resulted either from the US air raid itself or from secondary explosions of munitions stored by Shabaab at that location, the Africa Command added.
"We are deeply sorry this occurred," Africom leader US Army Gen Stephen Townsend said.
Admissions of civilian deaths caused by US military operations in Somalia reflect what Gen Townsend described as a policy of increased candour on Africom's part.
Since taking charge of Africom in July 2019, "I have placed additional focus on increasing the transparency of our process and status of closed and ongoing allegations," Gen Townsend said.
"There is no secret air or shadow war as some allege," the general added.
"How can there be when the whole world knows we are assisting Somalia in their fight against Al-Shabaab terrorists? When we publicly announce every single airstrike we conduct? When we publicly admit to our mistakes?"
Gen Townsend further declared: "It is important that our partners and the public understand our commitment to minimising collateral damage while conducting military operations. Where we come up short, we will admit it openly."
Allegations of civilian killings arise from reports by Africom itself, as well as through traditional journalism channels and social media, Africom said. Claims made by non-governmental organisations are also assessed, along with reports by individuals filed through a link on Africom's website.
A finding that an allegation is "unsubstantiated" is not intended to rule out the possibility that a civilian casualty may in fact have occurred in a particular incident, Africom cautioned.
An "unsubstantiated" determination is said to reflect Africom's assessment that a civilian death or injury either did not occur in the course of US military operations or that there is insufficient information to support an allegation.
A charge of civilian deaths or injuries is found to be "substantiated" when Africom has determined that US military operations had "more likely than not" caused such casualties, the Africa Command added.
Africom noted that its information collection efforts "are based on layered and reliable intelligence sources that are not available to the public to preserve operations security."
This may ultimately lead to perceived discrepancies between the command's findings and those of others, this initial quarterly report added.