UN Secretary-General Ban Ki-moon Saturday urged leaders of the world's 20 largest economies not to "balance budgets on the backs of the world's poorest people."

Ban made the appeal in a speech delivered at a Group of 20 (G20) working dinner in the largest Canadian city of Toronto.

"Let me emphasize this evening that, under any circumstances we must not balance budgets on the backs of the world's poorest people," Ban told the luncheon, quoted by a readout from his press office.

Citing figures from experts, Ban said: "Fifteen billion U.S. dollars is needed immediately for the 49 least developed countries."

Ban said that he understood concerns about rising budget deficits and public debt and that the severe fiscal problems faced by some countries required urgent action.

"In London and in Pittsburgh, we came together to avert a global depression," he said. "We succeeded. You showed great leadership. Our challenge now is different."

He said the UN's Global Pulse monitoring system showed that the poor and vulnerable need help to make ends meet, "now more than ever."

"We cannot depend on consumption-led recovery alone but also need to mobilize investments to ensure today's recovery and tomorrow's future growth," Ban said.

According to the readout, the UN chief identified agriculture, green recovery and health as three areas of high returns for investment by world governments.

"Let us be determined to turn these three areas of high-return investments into a reality," Ban said. "Tomorrow, the G20 can make clear its intention to do so."

He hoped that world leaders could turn their commitments into a bold, detailed and accountable plan of action to achieve Millennium Development Goals at a UN summit in New York in September.