The US defence secretary has met Qatar's leader during a visit to the Gulf state, home to the largest American airbase in the Middle East.

James Mattis said Saturday's meeting in Doha with Emir Sheikh Tamim bin Hamad Al-Thani aimed to "reinforce relationships" between the two countries.

"Relationships get better or weaker, and I'm committed to making it better from our side," Mattis told the Emir.

Their talks were expected to focus on the fight against the Islamic State of Iraq and the Levant (ISIL) group, the conflict in Syria, and the regional role of Iran, which Mattis has described as "destabilising".

Mattis was also due to hold dicussions with Khaled al-Attiya, Qatar's defence minister.

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.

US relations with Gulf Arab states became increasingly frayed during the presidency of 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 US-led invasion of Iraq in 2003, said in Israel on Friday there was "no doubt" Syria has kept some chemical weapons and warned President Bashar al-Assad's government not to use them.

Assad has said repeatedly that his forces turned over all chemical-weapon stockpiles in 2013, under a deal brokered by Russia to avoid threatened US military action.

Qatar plays a key role in regional politics and is home to the Al-Udeid airbase, which houses about 10,000 American troops.