US Secretary of State Rex Tillerson met Syrian opposition negotiators in Jordan on Wednesday for closed-door talks in the wake of a January peace summit hosted by regime backer Russia.

The opposition and Kurdish groups had boycotted the congress, held just days after a ninth round of United Nations-led talks in Vienna failed to yield progress towards ending Syria's devastating conflict.

The West views Russia's efforts for peace in Syria with suspicion, concerned that Moscow is seeking to sideline the UN process.

But the US acting assistant secretary of state, David Satterfield, said on Wednesday that the fallout had been limited.

“We got in the end a communique which validated the UN role,” Satterfield said before Tillerson's meeting in Amman. “So this game, this theatre that was Sochi... finally came out in a way that did no damage.”

Despite Moscow insisting Syrian society would be fully represented at Sochi, almost all of the 1,400 delegates were pro-regime.

They agreed to set up a commission to re-write the country's post-war constitution.

About 150 Syrian civil society groups accused the UN of rewarding Russia by dispatching its special envoy Staffan de Mistura to the congress.

Syria's war has killed more than 340,000 people and displaced millions since it began in March 2011 with the brutal repression of anti-government protests.

US pledges over $1 billion in annual aid to Jordan

The United States is to send aid worth over $1 billion a year to Jordan under a deal signed on Wednesday by US Secretary of State Rex Tillerson and Jordanian Foreign Minister Ayman Safadi.

The agreement, worth $1.3 billion (1 billion euros) a year from 2018 until 2022, boosts Washington's annual aid package to the key American ally by $275 million.

The first such deal between the two since Donald Trump took office as president, it commits the US to providing at least $750 million a year in economic support and $350 million in military aid.

Jordan is one of the world's largest recipients of American aid, having received over $20 billion since 1951. The kingdom also relies heavily on aid from the European Union and Saudi Arabia.

Its economy has been battered by the conflicts in neighbouring Iraq and Syria and its public debt stands at around $35 billion.

The United States has separately provided nearly $1.1 billion in humanitarian aid to support Syrian refugees in Jordan.

The kingdom hosts around 650,000 Syrians who have fled their war-ravaged country since March 2011, as well as a further 700,000 who entered Jordan before the conflict.

Amman says the Syrian crisis has cost it more than $10 billion.