US Secretary of State John Kerry on Thursday visited the sprawling Zaatari camp home to tens of thousands of Syrian refugees to see at first-hand the tragedy of the conflict.

Kerry first overflew the vast camp by helicopter, surveying hundreds of tents and trailers lined up on the desert sand about 20 kilometres (12.4 miles) from the Syrian border.

Kerry said his visit had "put a real human face" on the situation, describing his conversations with refugees and officials at the camp as "searing and unforgettable".