Getty Images
Now playing
01:37
Why Al-Shabaab is a growing threat (2015)
Now playing
03:44
Do the 1990s hold the key to sustainable websites?
Now playing
01:10
Watch SpaceX land its Mars rocket prototype for the first time
Now playing
05:52
The climate crisis is taking these farmers' most valuable resource
U.S. President Donald Trump works on his phone during a roundtable at the State Dining Room of the White House June 18, 2020 in Washington, DC. President Trump held a roundtable discussion with Governors and small business owners on the reopening of American's small business. (Photo by Alex Wong/Getty Images)
Alex Wong/Getty Images
U.S. President Donald Trump works on his phone during a roundtable at the State Dining Room of the White House June 18, 2020 in Washington, DC. President Trump held a roundtable discussion with Governors and small business owners on the reopening of American's small business. (Photo by Alex Wong/Getty Images)
Now playing
02:37
Facebook Oversight Board: Indefinite suspension of Trump's account is 'not appropriate'
Now playing
03:41
How technology at NASA helps guide Biden on climate
Now playing
01:42
A vaccine without needles? It's on the way
SAN FRANCISCO, CA - FEBRUARY 06:  Host Conan O'Brien speaks onstage during the 5th Annual NFL Honors at Bill Graham Civic Auditorium on February 6, 2016 in San Francisco, California.  (Photo by Tim Mosenfelder/Getty Images)
Tim Mosenfelder/Getty Images
SAN FRANCISCO, CA - FEBRUARY 06: Host Conan O'Brien speaks onstage during the 5th Annual NFL Honors at Bill Graham Civic Auditorium on February 6, 2016 in San Francisco, California. (Photo by Tim Mosenfelder/Getty Images)
Now playing
01:48
Conan announces his last show after 30 years in late night
Drew Angerer/Getty Images
Now playing
03:14
Will Trump be allowed back on Facebook? This board will decide
CNN
Now playing
05:16
WTF is a SPAC?
Now playing
01:51
A shortage of tanker truck drivers could cause stations to run out of gas
CEO at Verizon Media K. Guru Gowrappan appears at the 2019 Verizon Media NewFront on April 30, 2019 in New York City.
Noam Galai/Getty Images for Verizon Media
CEO at Verizon Media K. Guru Gowrappan appears at the 2019 Verizon Media NewFront on April 30, 2019 in New York City.
Now playing
02:47
Verizon sells off Yahoo and AOL
Warren Buffett, CEO of Berkshire Hathaway, attends the 2019 annual shareholders meeting in Omaha, Nebraska, May 3, 2019. (Photo by Johannes EISELE / AFP)        (Photo credit should read JOHANNES EISELE/AFP via Getty Images)
Johannes Eisele/AFP/Getty Images
Warren Buffett, CEO of Berkshire Hathaway, attends the 2019 annual shareholders meeting in Omaha, Nebraska, May 3, 2019. (Photo by Johannes EISELE / AFP) (Photo credit should read JOHANNES EISELE/AFP via Getty Images)
Now playing
02:40
Warren Buffett warns on US inflation
Now playing
01:00
Astronauts splash down after record-setting mission
ATLANTA - APRIL 30:  A Boeing 757 with a new Delta Airlines logo sits on the tarmac following the company's emergence from bankruptcy at Hartsfield Jackson International Airport April 30,2007 in Atlanta, Georgia. The 757 sports new branding that will appear on more than 900 planes, at airports and on advertising.  (Photo by Barry Williams/Getty Images) 757
Barry Williams/Getty Images North America/Getty Images
ATLANTA - APRIL 30: A Boeing 757 with a new Delta Airlines logo sits on the tarmac following the company's emergence from bankruptcy at Hartsfield Jackson International Airport April 30,2007 in Atlanta, Georgia. The 757 sports new branding that will appear on more than 900 planes, at airports and on advertising. (Photo by Barry Williams/Getty Images) 757
Now playing
02:28
Why Delta airlines is resuming selling middle seats
Now playing
02:50
Grocery chain says 'hero pay' forcing them to close stores

Story highlights

At least 25 Somali troops died in the attack on an African Union base in southern Somalia, military source says

Twelve Ugandan soldiers also were killed, a Ugandan military spokesman confirmed

Al-Shabaab appears to be shifting tactics in attacking more remote AU military bases

(CNN) —  

At least 37 African Union soldiers were killed in an attack by Al-Shabaab militants in southern Somalia on Tuesday, according to military sources in Uganda and Somalia.

A source from Somalia’s security services told CNN that at least 25 Somali government troops were killed in the fight. Twelve Ugandan soldiers also were killed, Uganda’s military spokesman Lt. Col. Paddy Ankunda confirmed.

The attack by the Somali terrorist group occurred on a remote African Union base in the town of Janaale. It was the largest loss sustained by the Ugandan contingent of the AU Mission to Somalia, or AMISOM, since 2012.

Ankunda told CNN by phone that 10 of the fallen soldiers’ bodies had already arrived back in Uganda.

Other Western sources said that as many as 50 AU troops were killed.

The 22,000-strong, United Nations-backed AMISOM force consists of troops from Uganda, Burundi, Kenya, Ethiopia, Djibouti and Sierra Leone. It has been dealt serious blows in recent months in Al-Shabaab attacks. The terror group seems to be shifting tactics, moving from smaller, asymmetrical suicide-bombing attacks on civilian, government and military targets to larger and more sustained operations solely focused on smaller, remote AU bases.

A member of Al-Shabaab told CNN that the battalion behind Tuesday’s attack is the same group of about 200 fighters who assaulted an Ethiopian convoy and a base predominantly held by Burundian soldiers in Leego, Somalia, in July. Those two attacks killed dozens of AMISOM troops. The fighters call themselves the Abu Zubair Battalion – named after the Al-Shabaab leader (a.k.a. Ahmed Abdi Godane) who was killed by a U.S. drone strike in Somalia in September 2014.

Al-Shabaab says the attack was in retaliation for alleged AMISOM attacks on civilians.

The United States on Thursday condemned Al-Shabaab for its “brutality in its depraved efforts to destabilize Somalia and deny the people of Somalia the peaceful, prosperous future they deserve.”

A Western source told CNN that the AMISOM had recently withdrawn their heavy weaponry from the Janaale base to be used elsewhere against the Islamic extremists.

The source says Al-Shabaab also destroyed two nearby bridges, trapping the Ugandan soldiers. Militants loaded a car with explosives to breach the perimeter of the base.

The source added that poor weather conditions prevented U.N. air assets from reaching the attack to help evacuate the wounded Ugandan soldiers.

The U.N. has been criticized for not providing funding for more air assets in the fight. The Ugandan military has had no troop transport or armed helicopters in Somalia. Because of damaged roads after more than 20 years of civil war, ground transportation is slow and unreliable, making it harder to respond quickly when remote bases such as Janaale come under attack.

Ethiopia and Kenya have attack and troop transport helicopters. Kenya also uses fighter jets to strike Al-Shabaab targets, and the U.S. uses drones at times to help with surveillance and launch missiles at high-value terrorist targets.

What is Al-Shabaab?

CNN’s Nana Karikari-apau contributed to this report.