SEOUL (AFP) - North Korea has cancelled this year's Wonsan Air Show, travel agents said on Monday (Aug 28), with reports suggesting that tightened travel restrictions by several countries due to nuclear tensions with Pyongyang could be behind the move.

The first-ever Wonsan International Friendship Air Festival was held in September last year, attracting hundreds of foreigners as the isolated North showcased a line-up of civilian and military aircraft.

The event had been planned to take place every two years, but North Korea said in March the first was so successful that it was bringing forward the second edition.

But the gathering, scheduled for late September, was cancelled by the North Korean Preparatory Committee due to "current geopolitical circumstances", specialist website NK News reported, citing an e-mail from the organisers.

It did not give specific reasons but cited "uncertainty" and "recently upgraded travel warnings" regarding trips to the North, the report said.

A US travel ban issued following the June death of US student Otto Warmbier, who was released in a coma after being held by Pyongyang for more than a year, will come into effect on Friday.

Britain updated its advice at the weekend to urge its citizens against "all but essential travel".

Japan's public broadcaster NHK cited diplomatic sources as saying the reason for the cancellation was unclear. It said there was speculation it could be part of the North's efforts to save fuel in the face of increasingly tough United Nations Security Council sanctions over its nuclear and ballistic missile programmes.

Last month, Pyongyang tested two intercontinental ballistic missiles that appeared to bring much of the US mainland into range.

Oil is not currently subject to sanctions but fuel prices jumped in Pyongyang earlier this year, prompting suggestions that the rise was driven by a fear of restrictions being imposed.

Specialist tour agencies which had scheduled trips to the air show said they had cancelled them.

"We have indeed been informed that the Wonsan Air Festival has been cancelled," Mr Stefan Goossens, a tour organiser at Netherland-based 4Aviation, said.

Mr Rayco Vega at China-based KTG DPRK Tours & Information, said that the agency has scrapped its plans.