"Today there were massive attacks in Jabaliya when we were there. We saw that there were numerous fires once the white phosphorus had gone in."
International law permits the use of white phopshorus as an "obscurant" to cover troop movements and prevent enemies from using certain guided weapons, but its use is controversial as it can injure people through painful chemical burns.
"Even if they are using it as an obscurant, they are using it in a very densely populated area," Garlasco said.
"The problem is it covers such a wide area that when the white phosphorus wafers come down, over 100 in each artillery shell, they burn everything they touch and they don't stop burning until they are done.
"You are talking about skin damage, potentially homes going on fire, damage to infrastructure."
Human Rights Watch said that it believed the use of the chemical in Gaza violated the requirement under international humanitarian law to take all feasible precautions to avoid civilian injury and loss of life.
The Israeli military has previously denied using white phosphorus during the 15-day offensive in the Gaza Strip, but has said that any munitions that it does use comply with international law.
Israel used white phophorus during its 34-day war against Lebanon's Hezbollah movement in 2006, while the United States used it during the controversial siege of the Iraqi city of Fallujah in 2004.