TEL AVIV, Israel — International aid groups are periodically called upon to provide food, water, medicine and other vital supplies to people caught in war zones. But getting that critical aid to a population on the run and under fire presents enormous challenges, humanitarian groups say, and the situation in Gaza is proving particularly difficult.

"We are being slammed left, right and center," says Juliette Touma, a spokesperson for UNRWA, the United Nations relief agency that aids Palestinians.

"This is very different than any other conflict or war we've had to manage," Touma says, adding that it is the largest humanitarian response operation in UNRWA's nearly 75-year history.