nearly three quarters of the money spent on air fares at UN headquarters goes on business class.
That is "clearly unjustifiable," said Joseph Torsella, the US envoy who since 2011 has been leading a US war on "waste" at the UN.
Rules on business class travel are "out of whack" and the failure to enact "common sense and overdue reforms is creating a system that is ripe for abuse," the US envoy for UN management and reform told AFP.
The United Nations spent at least $769 million of its general budget of more than $5 billion in 2010-11 on moving officials and staff around the world, according to UN figures.
The peacekeeping department, which has its own budget, spent another $200 million.
Some $54 million of the $74 million of air tickets bought at the main headquarters in New York and Geneva were business class. Diplomats say the figure is probably much higher as no clear figures have been given.
"There are are a series of loopholes that are just on the face of it crazy," Torsella said.
Most UN staff travel under a 1990 system where a lump sum of 75% of the full economy class fare can be given. UN auditors estimate that this now costs 83% more than current regular fares.
The UN has about 30 different outside travel agencies and so does not get economies of scale and hardly uses online booking, Torsella said.
UN staff can claim business class for any trip of nine hours, even if they arrange a stopover to make it longer. The US government only allows business class for 14 hours in the air. Torsella also questions giving UN staff a daily living allowance for time spent flying.
"When you look at these things, the business class use, the lump sum, the daily subsistence allowance, these loopholes, the conclusion seems inescapable that taxpayers are not getting value for their dollars and neither is the UN," said the US official.
UN leader Ban Ki-moon last year recommended just "encouraging" greater use of teleconferences, that anyone going to a training course should fly economy and ending the daily allowance for time in the air.
These ideas have "languished" because of wider political battles between the 193 members states on the UN General Assembly's budget committee which sets the rules and is meeting again this month, said Torsella.
He points out that taking 10 per cent off the travel bill could buy 15.4 million anti-malaria nets.
"Pick your continent and pick your issue and ask is the UN doing absolutely everything that needs to be done, the answer is surely going to be no," he said.
"This is as clear an example as you can get of a place where most member states should be able to make common cause, where the UN ought to be able to save money and improve service and where notably it is the General Assembly not letting the secretary general do the right thing."
Since Torsella started his job, the global body has frozen salaries for its 10,000 New York staff. This month he called for drunken diplomats to be banned from the UN's hard-fought, fiercely political budget talks.