Irish aid agency GOAL is warning that the damage caused by Cyclone Idai is being hugely underestimated across Southeast Africa.
The charity's staff on the ground have heard reports of whole villages being destroyed with thousands of bodies.
The UN says the final casualty figure will only be known once the floods have receded.
GOAL's Chris Boucher has been to Malawi where he was part of the rapid response team distributing food, water and blankets.
He says the full extent of the disaster has yet to be seen.
"Up to 2.5 million people are affected by this disaster so far," said Mr Boucher.
"Many hundreds of thousands of people have entirely lost their homes. They've lost their crops and their ability to drive a livelihood.
"800 people are confirmed dead so far. But truthfully, we don't actually know the full extent of this yet.
"So there are many areas which are still inaccessible. So the true extent of this crisis is only slowly becoming apparent."