US defence chief holds security talks with Qatar
The regional tour has included stops in Saudi Arabia, Egypt, Israel, with Djibouti set to be Mattis' next stop on Sunday.
The Pentagon chief met with Emir Sheikh Tamim bin Hamad al-Thani and Defence Minister Khaled al-Attiya during his brief visit to Qatar, which aims to improve ties with US President Donald Trump's administration.
Qatar, a key US ally in the region, is also home to the largest US airbase in the Middle East.
Analysts say that the talks likely focused on the fight against the Islamic State group, the Syrian crisis, and Iran's influence on the region, which Mattis has described as "destabilising".
US ties to Qatar and other Gulf became strained during the presidency of Barack Obama, who many in the Middle East saw as too hesitant on the civil war in Syria, as well as overly friendly with Iran.
Last year, the US signed the nuclear accord with Tehran, which resulted in the lifting of sanctions in exchange for a pledge that Iran would not pursue nuclear weapons capabilities. Trump's open criticism of the deal has been welcomed by Gulf leaders, who view Iran as a key threat.
On Syria, Mattis said that there was "no doubt" that the regime of President Bashar al-Assad has kept some chemical weapons. He warned Syria not to use them.
Doha has long supported Syrian rebel groups, while also acting as a key negotiator in brokering deals between regime and opposition forces. This includes a recent deal to evacuate thousands of Syrians from besieged towns.
The US and Qatar also share close economic ties, with national carrier Qatar Airways announcing in October the acquisition of up to 100 Boeing aircraft worth $18.6 billion.