US troops took part in a raid on the Islamist al-Shabab group alongside Somali troops, American officials say.
Al-Shabab earlier said it had “fought off” an attack by foreign forces but US officials said several militants were killed in the raid early on Wednesday.
Several countries have been supporting Somali officials in their efforts to combat al-Shabab’s insurgency.
The assault comes after the US said it conducted air strikes on Saturday on an al-Shabab training camp.
Al-Shabab said one of its fighters died in the battle, while a Somali official put the number of militants killed at 15.
A Somali intelligence official told the Associated Press news agency that Wednesday’s raid had targeted a particular “high-profile” individual, who had died in the attack.
The African Union mission in Somalia has not commented on the latest raid.
A more aggressive effort – Tara McKelvey, BBC White House reporter
For some, a US special forces raid in Somalia is a sign the US is carrying out a more aggressive campaign against militants – while others say it’s just business as usual, at least for the US.
However you see the latest operation, one thing is clear. US forces are hitting militant groups in Somalia hard. The US carried out an air strike on Saturday, reportedly killing more than 150 al-Shabab fighters.
I asked White House spokesman Josh Earnest if the air strike, a massive one by any measure, meant the US was pursuing a different strategy. He didn’t confirm anything about Somalia. But he did say the situation was fluid.
“The threat has changed,” he said, describing “different kinds of terrorist groups operating in different parts of the world”.
So there’s no official word about a change in strategy. But a more aggressive effort towards militants in Somalia seems under way.
The operation targeted an al-Shabab base near the town of Awdhegele, about 50km (30 miles) south of the capital Mogadishu.
“They were masked and spoke foreign languages which our fighters could not understand,” al-Shabab spokesman Sheikh Abdiasis Abu Musab told Reuters news agency.
He said the helicopters landed on the banks of the River Shabelle and commandos from the aircraft advanced on the base.
They carried rocket launchers and M16 rifles, he told Reuters, citing a weapon used by US forces.
Awdhegle District Commissioner Mohamed Aweis told the BBC that the “helicopters were giving air support to an infantry special forces”.
Residents said there had been a gun battle, and that the group had sealed the area so it was not possible to confirm if there were any casualties.
Al-Shabab is battling the Western-backed Somali government to establish Islamic rule in the country. Local forces fighting the group have been backed by an African Union mission made up of forces from several African nations.