Air traffic controllers in the United States warn that flying is less safe because of the partial government shutdown which means they are working for no pay.

The warning comes as their counterparts in New Zealand say the shutdown meant the aviation industry had reached a ''frustrating and historical new low''.

The New Zealand Air Line Pilots' Association represents air traffic controllers here and its president Tim Robinson said, "We salute our US colleagues and can only imagine the uncertainty and financial concerns they, their families and others in the wider aviation community affected by the shutdown are going through."

Today the US National Air Traffic Controllers Association said that flying was less safe today than it was a month ago.

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The shutdown is into its fourth week and the association's executive vice president Trish Gilbert said there were barebones crews of controllers who must work because they are deemed essential workers.

"We have controllers there doing what they do very, very well — but how long can you expect them to do it without all of the systems behind them to keep the system safe? And the planes in the air?" she told CNN.

The Federal Aviation Administration will soon recall more than 3400 workers who had been furloughed, but who are now needed for the "operational safety of the entire national airspace".

Although there haven't been delays reported due to the pressure on controllers, shortages of other federal workers affected such as Transport Security Administration employees had led to longer queues at US airports.

Robinson, a pilot who flies to the US, said his association had a close relationship with the controllers' union in the US.

''In New Zealand where ATCs currently face their own workload challenges and resourcing challenges, we consider NATCA a true leader amongst global aviation trade unions, by demonstrating their ability to function flawlessly to support and encourage their members in their greatest time of need."

It was understood the total controller headcount was at the lowest for 30 years and the traffic level at its highest.

"The training college is shut down, the safety office is closed, and all new projects are on hold. All non-essential and administrative staff have been sent home," said Robinson.

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"The effects of this shutdown will be felt at the FAA for years to come, it will delay implementation of new technology and systems, stall construction and refurbishment, and cripple the training system as unpaid trainees leave to take alternative work."

Already the shutdown has hit airline profits.

Delta Air Lines said the shutdown was eroding sales and will hurt its ability to raise ticket prices.

It said revenue would be cut by US$25 million (NZ$37m) this month as government workers and contractors travel less.

Southwest Airlines' long-planned flights to Hawaii are on hold too because FAA workers who need to sign off extended overwater flights are on furlough.