The number of Syrians who have fled their country after six years of war has surpassed the 5 million mark, the U.N. refugee agency said Thursday.

UNHCR announced the milestone a year after participating countries at a Geneva conference pledged to "resettle and facilitate pathways for 500,000 refugees" from Syria - but that only half of those places have been allocated so far.

"We still have a long road to travel in expanding resettlement and the number and range of complementary pathways available for refugees," said U.N. High Commissioner for Refugees Filippo Grandi. "To meet this challenge, we not only need additional places, but also need to accelerate the implementation of existing pledges."

Agency spokesman Babar Baloch said that no specific incident prompted the crossing of the symbolic milestone, and that one year ago, the figure was 4.8 million. The agency estimates another 6.3 million people have been internally displaced.

Turkey, which has taken in the most Syrians by far, saw an increase of 47,000 Syrian refugees since February - bringing its total to 2.97 million, Baloch said. Lebanon and Jordan have taken in hundreds of thousands of Syrians.

The United States has the world's largest resettlement program, and has pledged to make 64,000 places available for Syrians, Baloch said. The Trump administration has sought to reduce the program, though Baloch said the agency's efforts to help resettle Syrians in the U.S. are continuing amid U.S. court battles over the issue.