Separate attacks have targeted a US military base in the Somali town of Baledogle and a European military convoy in the capital, Mogadishu.
A bomb attack was followed by a gun battle at a base operated by US special forces on Monday, a security source told Reuters news agency.
The Somali armed group al-Shabab claimed responsibility for the attack at the Balidogle base located in the Lower Shabelle region, about 100km (60 miles) west of Mogadishu.
"Two heavy explosions occurred, the first one bigger than the other. There was also a heavy exchange of gunfire after the blasts but we don't know about the details," Mohamed Adan, a Somali elder close to the scene of the attack, told AFP news agency by phone.
Al Jazeera's Malcolm Webb, reporting from neighbouring Kenya, said the first attack targeted the base, housing US troops and Somali soldiers, used to carry out drone attacks on the al-Shabab group.
In a separate incident, a security official said European Union (EU) advisers training the Somali National Army were attacked using a car bomb in Mogadishu.
Italy's defence ministry in a statement confirmed that an Italian military convoy was hit by an explosion.
No injuries have been reported so far, the defence ministry said.
"There was a car bomb targeting the EU military advisers along the industrial road. A vehicle loaded with explosive was rammed onto one of the convoy vehicles and there are casualties," said Omar Abikar, a Somali security officer.
Al Jazeera's Webb added that an improvised device was used to target the Italian convoy, which has been guarding the EU team.
The EU has been training the Somali national army, among other activities, he said.
"The attack happened as Mogadishu was under a lockdown due to a high-level meeting at the UN headquarters. And it's believed the convoy may have been coming to or from that meeting," Webb said, reporting from Nanyuki.
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