Passengers will soon be allowed to bring small pocket knives on board airplanes, the US Transportation Security Administration has said.

Such items have been banned since the 11 September attacks in 2001.

The decision has angered flight attendants, who said the move would endanger passengers and crew.

The TSA said it would allow knives with blades that are 6cm or less in length and less than 1.25cm wide from 25 April.

Other items that will be allowed on board again as part of a passenger's carry-on luggage include billiard cues, ski poles, hockey sticks and lacrosse sticks.

Items such as razors, box cutters or knives with a fixed blade are still not allowed on board.

TSA spokesman David Castelveter said the decision was made to bring US regulations more in line with International Civil Aviation Organisation standards.

He said they would also help provide a better experience for travellers.

"This is part of an overall Risk-Based Security approach, which allows Transportation Security officers to better focus their efforts on finding higher-threat items such as explosives," he said.

The Flight Attendants Union Coalition, which represents nearly 90,000 flight attendants from carriers across the US, called it a "poor and short-sighted decision".

"As the last line of defence in the cabin and key aviation partners, we believe that these proposed changes will further endanger the lives of all flight attendants and the passengers we work so hard to keep safe and secure," the coalition said in a statement.

Mr Castelveter said the TSA had implemented a number of safety measures, including reinforced cockpit doors, allowing some pilots to be armed and federal air marshals on board airplanes.

He said those measures would help ensure the safety of the passengers and crew.