Up to 80,000 people could be affected, according to the International Federation of Red Cross and Red Crescent Societies.
Tsunami waves following the eruption also wreaked significant damage.
“Amid the chaos and the uncertainty, we pray for you and we stand in solidarity with you amid a terrifying disaster,” said Sauca. “We pray for God’s mercy on you and on the rescue workers and first responders at the scene. We pray that God might guide the churches responding to the needs of the people as those needs change in the coming days.
“We assure you that our solidarity and prayers will be ongoing as you continue to cope with the devastation of this disaster,” said Sauca.
Locals say Tonga is coated in a layer of volcanic ash, and the dust was reportedly contaminating water supplies. Tonga's internet and phone lines went down, making the island's 105,000 residents almost entirely unreachable, though communications were gradually returning.