Cameron flew in from Lebanon where he met Syrian refugees hours after appointing a minister to oversee the resettlement of Syrian refugees in Britain.
In Jordan, he toured Zaatari, a sprawling desert camp in the north of the kingdom which is home to nearly 80,000 Syrian refugees.
Lebanon and Jordan have complained that their resources have been stretched to breaking point by the influx of refugees from Syria, and UN agencies have repeatedly appealed for donors to help them aid needy Syrians.
Syria's civil war broke out four and a half years ago when President Bashar al-Assad brutally cracked down on peaceful protests against him and people took up arms.
It has claimed more than 240,000 lives and driven nearly half of Syria's people from their homes. Some four million people have fled abroad, primarily to neighbouring Jordan, Lebanon and Turkey.