The United Nations has said it estimates 3.7 million people are in acute need of food in South Sudan as a result of the civil conflict there.
Violence broke out in South Sudan on 15 December, starting as violence between rival army factions.
It has now killed thousands of people and displaced around 860,000.
Mr Lanzer said it had also had profound effects on the country’s economy. “Largely because markets have been disrupted, people have been living under extreme duress, people aren’t able to move as they normally would,” he said.
“Nobody in mid-December… could have foreseen the scale of the emergency that now faces us. We are doing everything we can to avoid a catastrophe,” he added.
The number of those needing food represented around a third of South Sudan’s population, he said.
He said that in the city of Malakal, some civilians had stormed a warehouse where aid was being kept and “helped themselves”.
“Most of the looting was done by people who were so desperate for the aid that they simply couldn’t wait,” he said.
He said over that of 863,000 people who have been displaced; 740,000 of them are still in South Sudan and the others have left for neighbouring countries.