MAE SOT, Thailand — A new crackdown on online scam centers has led to over 7,000 people from around the world being held in a Myanmar border town awaiting repatriation, and those helping them say the unprecedented number is straining the resources of Thailand just across the border and leading to delays.

The crackdown coordinated among Thailand, Myanmar and China follows Thai Prime Minister Paetongtarn Shinawatra’s visit to Beijing this month, where she told Chinese leader Xi Jinping that…