US Defence Secretary Jim Mattis met Qatar's emir Saturday during a visit to the oil-rich Gulf state, home to the largest US air base in the Middle East.
The visit to Doha is part of a regional tour that has included stops in Saudi Arabia, Egypt and Israel. Mattis is scheduled to visit Djibouti on Sunday.
The Pentagon chief met with Emir Sheikh Tamim bin Hamad Al-Thani and was also due to hold talks with Defence Minister Khaled Al-Attiya during his brief visit to Qatar, which aims to improve ties with US President Donald Trump's administration.
Talks were expected to focus on the fight against the Islamic State group, the Syrian crisis, and the regional role of Iran, which Mattis has described as "destabilising".
Washington's ties to Gulf states became increasingly frayed during the administration of president Barack Obama, whom leaders saw as too reluctant to intervene in the civil war in Syria and overly friendly with regional rival Iran.
Mattis, who commanded troops during the 2003 invasion of Iraq, said in Israel on Friday that there was "no doubt" Syria has kept some chemical weapons and warned President Bashar al-Assad's regime not to use them.
A small but strategic state in the Gulf, Qatar plays a key role in regional politics.
It is home to the Al-Udeid air base which houses around 10,000 US troops.
A longti me supporter of Syrian rebel groups, Qatar was also a key negotiator in brokering a recent deal to evacuate thousands of Syrians from besieged towns.
Rival Iran, an ally of Assad, played a crucial role in the evacuation deal, which according to sources in Baghdad included the release of a Qatari hunting party kidnapped in southern Iraq in 2015.
Washington has previously accused Doha of not doing enough to fight extremist organisations such as the Islamic State group. Qatar has denied the accusations.
The two countries have close economic ties, with national carrier Qatar Airways announcing in October the acquisition of up to 100 Boeing aircraft worth $18.6 billion (17.3 billion euros).