Firefighters cut through a window to reach the trapped pair, who were attempting to ascend to the top of the skyscraper about 12.45pm on Wednesday, US time, when a cable came loose.

They were left hanging at a sharp angle, 240 metres above ground on the south side of the tower, which is the the tallest skyscraper in the US and the fourth tallest in the world.

About 100 firefighters were involved in the complex rescue operation in lower Manhattan. It took rescuers nearly an hour to reach the two men, who suffered mild hypothermia, said New York Fire Commissioner Daniel Nigro.

Rescuers cut a diamond-shaped hole out of three layers of glass on the 68th floor and brought the washers inside the building.

Before they were recovered, firefighters had suspended a cable to secure the dangling platform and communicated via radio with the two men, who were described as experienced window washers.

Ambulances, fire trucks and helicopters were dispatched to the scene and the area around the building was cordoned off.

The window washers were wearing safety harnesses and therefore were not in grave danger, authorities said, while forecasters reported light winds of 11km/h.

Before the pair were rescued, New York Police Department counter-terrorism spokesman John Miller estimated the platform was dangling at a 65-degree angle, according to the New York Post.

He spoke briefly about the window-washing incident at a New York City Council hearing.

"Ironically, as we sit here talking about the coordination between agencies and emergency management in the city of New York, at the World Trade Centre on the 69th floor, two workers are dangling at a 65-degree angle after the scaffolding partially collapsed," he said.

He said emergency services were "working together to resolve this issue with lives that are very literally and figuratively hanging in the balance."

Fire crews cut through a window to reach the pair, who were not believed to be injured. They were believed to have finished cleaning the building's windows and were trying to reach the roof when the platform collapsed.

The first tenants moved into One World Trade Centre last week, more than 13 years after the original World Trade Centre was attacked on September 11, 2001.

Megan Levy, AFP