North Korea has cancelled this year's Wonsan Air Show as reports suggest tightened travel restrictions by several countries due to nuclear tensions with Pyongyang could be behind the move. <br/><br/>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. <br/><br/>The event had been planned to take place every two years, but North Korea said in March the first was so successful that it is bringing forward the second edition.