Camp Lemonnier, home to some 4,000 US soldiers and contractors, is vital to US military operations in Somalia against militant groups like Al-Shabaab, and also provides support for US operations in Yemen, where special forces regularly carry out drone strikes against Al-Qaeda in the Arabian Peninsula.
China is also in the process of establishing its first overseas military base in the small port country just a few miles from the US camp, which has raised concern in Washington.
"They have always proven that they will stand up when it is time to stand against something like this."
The US backs France's Operation Barkhane, under which its military is fighting Islamists in five countries across the Sahel region -- Mauritania, Mali, Chad, Niger and Burkina Faso -- alongside African allies.
During the visit to the former French colony Mattis met with President Ismael Omar Guelleh as well as with General Thomas Waldhauser, commander of US troops in Africa.
Another senior defence official, also speaking on condition of anonymity, played down any concerns about China's base construction.
"At this point I don't see why we should not be able to comfortably coexist with the Chinese presence, the way we do with the Japanese, the French..." the official told reporters last week.
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