American cyclists have apologised for embarrassing their Chinese hosts by wearing face masks when they arrived at Beijing Airport.

The four cyclists had their faces covered by black breathing masks to combat Beijing's notorious smog when they arrived yesterday, but have since written to organisers saying sorry for their actions.

Jim Scherr, chief executive of the United States Olympic Committee, said: "The athletes regret that action and have written an apology to BOCOG on their own behalf.

"They now realise and understand how their actions were perceived by the host nation and by the organising committee.

"It was not the most opportune time for them to wear these masks, they were overly cautious.''

Footage of the cyclists - Mike Friedman, Bobby Lea, Sarah Hammer and Jennie Reed - was shown around the world.

On the same day as they arrived, the International Olympic Committee insisted the Chinese authorities anti-pollution measures - shutting hundreds of factories and keeping cars off the roads - were working.

Prof Arne Ljungqvist, head of the IOC's medical commission, said wearing the masks had been "unnecessary".

He added: "I would not wear one myself whether I was an athlete or not."