With close to one million people displaced since December by a Russian-backed regime offensive on the Idlib region, overcrowded settlements are teeming with fresh arrivals.
Many are still sleeping rough in what has been bitter cold.
Medical facilities have been targeted during the latest bombing campaign, further reducing the capacity of a health system ravaged by nearly nine years of conflict.
Unable to provide services from government-held territory inside Syria, the WHO provides cross-border assistance to rebel-held Idlib via Turkey, Halldorsson told AFP.
Health personnel are being trained and "laboratories in both Idlib and Ankara are being prepared and stocked to safely test and diagnose the virus", he added.
A Russian-Turkish ceasefire deal went into effect on Friday, bringing relative calm to Idlib for the first time in months.
But many fear the fighting will eventually resume, in a further challenge to efforts to prevent a COVID-19 outbreak.
Misty Buswell of the International Rescue Committee said the situation in Idlib was "especially ripe for a spread" of the virus.
"An outbreak would be devastating for thousands whose health status is already compromised due to lack of sufficient food, clean water and exposure to cold weather," she told AFP.