Beijing was finally experiencing clear skies today after city authorities apparently used seeding technology to disperse the smog. 

The report came as a strong 6.0 magnitude earthquake rocked the western Chinese provinces of Sichuan and Gansu today, near  the site of May's devastating quake that killed at least 70,000 people, the U.S. Geological Survey said.

Just hours earlier, the Olympic torch had passed through the area. Beijing organizers rescheduled the torch's run through Sichuan to support relief efforts, and the two days of events became the final stops on the flame's global tour before Beijing and Friday's opening ceremony.

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Anti-smog pellets fired in Beijing may have helped clear the air

Thick smog seen on the street of the Olympic Green in Beijing yesterday

There were no immediate reports of casualties or damage from the 6.0-magnitude quake, whose epicentre was 1,253 km (778 miles) southwest of Beijing. The magnitude of May's quake was 7.9.

A shallow quake of today's magnitude is capable of significant property damage.

In the meantime, Olympic chiefs remained more concerned with smog, fearing the high level of pollution, which is 10 times that in London, would cast a pall over the Games.

British athletes have been training in anti-smog masks. Poor conditions pose the greatest risk to endurance athletes such as world marathon record holder Paula Radcliffe, an asthmatic.

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Performers dance in traditional attire at a ceremony welcoming the torch in earthquake hit Sichuan province yesterday - just hours before an aftershock hit today

Such has been the concern about the smog that men's marathon champion, Ethiopian Haile Gebrselassie, withdrew earlier this year citing air quality.

But with clearer skies came murky accusations of gene doping, as a leading British scientist warned athletes may be injecting themselves with "super DNA".

This month's Games are believed to feature a new generation of "genetically modified" athletes whose performance has been improved by the injection of foreign DNA.

It could be the first Olympics to be tainted by "gene doping", the latest and most dangerous threat to drug-free sport.

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Tan Guoqiang, a school teacher at a school that was destroyed in the May earthquake, holds the Olympic torch as the last runner of the torch relay in Sichuan province yesterday

The process, placed on the list of banned substances and methods by the World Anti-Doping Agency in 2003, involves genes being inserted into muscle or bone cells and their proteins fed directly into the tissue or red blood cells. 

This is usually done by injecting, or sometimes inhaling, the necessary DNA.

Dr Andy Miah, in Beijing conducting research during the Games, said: "Gene doping is the next major headache for the world of sport.

"In 2004, people were starting to talk about its use at the Athens Olympics. This year in Beijing, the case is even stronger that this will be the first genetically modified Games. Many scientists will say it's still not possible, but I'm not taking this for granted. We need to assume that it's happening. It's already feasible."

Dr Miah's allegations will cast yet another unwelcome shadow over preparations for the 29th modern Olympics, which begin on Friday.

He added: "There is no other technology that is likely to change the Olympics than gene doping. It's not possible to detect and there's a good chance that it will never be detectable in any meaningful sense.

A DNA helix. Gene doping is said to be the next 'major headache' for the world of sport

"This forces the world of sport to reconsider what it does about testing. It's time for their plans to change. It's time for the era of human enhancement to take full effect in the Olympics."

Gene doping, to date, has been considered by most experts to be a hypothetical threat.

Many believed it would not register on the Olympic radar until at least the 2012 Games in London.

Dr Miah's claims will cause the WADA and the International Olympic Committee serious concern.

Dr Miah is a researcher in bioethics at the University of the West of Scotland and the Institute for Ethics and Emerging Technologies in the US. He is also the author of several books on the subject.

He added: "London 2012 should be watching Beijing very carefully to see what's possible. There has never been a 'clean' Olympics.

"The main problem for sports is that there are so many technologies that are under the radar of anti-doping, that its policies do little more than to point us to successes of anti-doping testing."

This development comes just weeks after a documentary revealed that a hospital doctor in China was prepared to give illegal performance-enhancing gene therapy treatment to an Olympic swimmer.

The doctor was caught on camera by a German television investigator saying that he wanted £12,000 for a two-week treatment that would help to strengthen the fictitious American swimmer's lungs.

The documentary, broadcast by ARD on Germany's main channel this month, showed the head of a hospital gene therapy department being approached by a fictitious swimming coach seeking stem-cell treatment.

The doctor replied: "Yes. We have no experience with sports people here, but the treatment is safe and we can help you."

Asked how it would work, the doctor said: "It strengthens lung function and stem cells go into the bloodstream and reach the organs. It takes two weeks.

"I recommend four intravenous injections ... 40 million stem cells or double that, the more the better. We also use human growth hormones, but you have to be careful because they are on the doping list."

Frederic Donze, of WADA, told the Evening Standard in Beijing today: "We have been preparing for gene doping since 2002.

"We have to believe that athletes will try anything to get an edge and this might occur at the Olympics and we work on that basis."

He said that the WADA had called together experts at a meeting in St Petersburg recently to draw up plans to combat the new drugs menace.