Hong Kong - Customs officials were on Friday stepping up the number of random checks on vehicles and boats coming into Hong Kong after claims that an outbreak of bird flu was caused by infected fertilized eggs smuggled from China.

A government spokesman said officers would target vehicles crossing the border while marine police would increase checks on vessels in Hong Kong's waters.

The move follows the cull of 75,000 chickens at a farm in the Yuen Long district where the outbreak of the deadly virus was detected early this week in 200 dead birds and at a nearby wholesale market in Cheung Sha Wan.

Officials confirmed Thursday the deaths had been caused by the H5N1 strain of the virus that jumps more easily from chickens to humans and has been responsible for 246 deaths worldwide since 2003, according to World Health Organization statistics.

Reports in the Hong Kong media quoted trade sources as saying the current outbreak had been caused by smuggled fertilized eggs containing chicken embryos infected with the virus.

The farm owner has insisted he has not taken any smuggled eggs and officials say there is no evidence in the farm's records to suggest he did.

However, Health Secretary York Chow said more checks would be carried out in view of the claims of smuggling.

"I think theoretically this can happen but we do not have any evidence so far," he said.

Six people died and 12 others were infected in an outbreak of bird flu in Hong Kong in 1997 that led to the culling of 1.2 million birds. Millions more birds were slaughtered in outbreaks in 2001 and 2002.

Hong Kong subsequently implemented strict controls on markets and chicken imports and escaped any human infections when bird flu swept the region in 2006.

Experts have repeatedly warned that the H5N1 strain of bird flu threatens a global pandemic if it mutates into a form that is more easily transmitted between humans.