The United States envoy for Syria on Saturday said Damascus's interim President Ahmed al-Sharaa is to travel to Washington to sign an agreement to join an international US-led alliance against the Islamic State.

Asked by reporters on the sidelines of the Manama Dialogue in Bahrain whether Sharaa would head to Washington this month, Tom Barrack said "yes", adding that Sharaa would "hopefully" sign up to the Global Coalition to Defeat Isis.

Barrack said Washington was aiming to recruit Syria to join the coalition Washington has led since 2014 to fight against Islamic State, the militant group that controlled around a third of Syria and Iraq at its peak between 2014 and 2017.

"We are trying to get everybody to be a partner in this alliance, which is huge for them," Barrack said.

It would be the Syrian leader's first visit to Washington and his second visit to the US after a landmark UN trip in September where the former jihadist became the first Syrian president in decades to address the General Assembly in New York.

Formerly affiliated with Al-Qaeda, Sharaa's group, Hayat Tahrir al-Sham, was de-listed as a terrorist group by Washington as recently as July.

In May, the interim leader, whose Islamist forces ousted long-time ruler Bashar al-Assad late last year, met US President Donald Trump for the first time in Riyadh in an historic visit that led to the American leader vowing to lift economic sanctions on Syria. (AFP)