WASHINGTON (AFP) - The US ambassador to Syria, Robert Ford, who has been Washington's main pointman in efforts to end the war, working with opposition leaders battling President Bashar al-Assad, stepped down Friday.

"Robert Ford is retiring from the foreign service today after nearly 30 years of distinguished service," State Department spokesman Jen Psaki told reporters.

She admitted that Mr Ford, who over the past three years has built up extensive contacts with the opposition leaders and was instrumental in helping to bring them to the Geneva peace talks, would be a loss.

"His extraordinary leadership has guided our response to one of the most formidable foreign policy challenges in the region," Ms Psaki said. "From the outbreak of the crisis, Ambassador Ford has worked tirelessly in support of the Syrian people in their pursuit of freedom and dignity."

Mr Ford left Damascus in 2011 when Washington decided to close the embassy as the uprising which began in March that year descended swiftly into a bloody civil war.

Since then he has shuttled between the United States and Turkey, spending hours huddled with opposition leaders based out of Istanbul as he sought to help them form a more cohesive and inclusive body.

Mr Ford has also spoken passionately and angrily about the mounting atrocities in the war which will enter its fourth year next month, and in which more than 136,000 people have been killed and millions displaced.

"There's no question that his departure is a loss, not just because of his contacts, but because of his expertise, because of his knowledge," Ms Psaki said.

She announced that for the time being as the White House mulls a replacement for Mr Ford, Deputy Assistant Secretary Lawrence Silverman would take up the helm.

"There will be a continuity, given that there are a range of officials who will still be in place," Ms Psaki said. "Obviously, part of what I'm sure will be looked at is the role that the next person will play in terms of their engagement with the opposition."

The UN-led Geneva II peace talks broke down on February 15 and no new date has yet been set for them to reconvene.

But Ms Psaki insisted that as the United States seeks to prepare for "a new Syria", and that Mr Ford's "legacy will guide our efforts to support Syrians and lay the foundation for a more hopeful future."

"The president and the secretary (John Kerry) of course are both incredibly grateful for his service," she added.