Mr Arsenault, who is Unicef's head of global emergencies, said the situation was dire, not only for the families who fled their homes but also for communities struggling to support them.
Hundreds of thousands of people are living in camps which are now overflowing but they represent a small fraction of the problem in Pakistan.
More than 80% of the displaced are far less visible, dependent on local hosts.
Some communities have doubled or tripled in size. Local homes are bursting and resources depleted.
Mr Arsenault said school buildings were now being used to house people with nowhere else to go.
Almost 4,000 schools have become camps, increasing by about 200 schools a week.
He said pressure was growing with monsoon rains and the planting season both about to start.
"If they [the displaced people] are not able to go back for the agriculture season they are going to miss the year and that is a critical factor," Mr Arsenault said.
Some people have returned but many say they are still not convinced the fighting is over.
Unicef has called on the government to give formal written guarantees of safety. It also wants basic services restored.
That needs a massive investment which so far foreign governments have been slow to make.
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