ATHENS, Aug 20: United States swimming sensation Michael Phelps took centre-stage at the Olympics on Friday by bagging his fifth gold medal of the Athens games as two new drugs cases emerged.

Phelps stayed on course to finish the Olympics with a remarkable six gold medals by producing a lung-bursting late surge to snatch victory by his fingertips in the 100m butterfly.

The 19-year-old pipped team mate Ian Crocker, the world record holder, at the touch by just four-hundredths of a second. "Having a long torso I guess helps," Phelps said. "The strategy was just to try to stay relaxed for the first 50 meters."

Phelps joined Mark Spitz as the only man to win four individual gold medals at one Olympics. Spitz accomplished the feat in Munich in 1972 as part of a seven-gold haul.

With seven medals overall and the medley relay still to be come, Phelps is closing in on matching the record for most medals at a single Games in any sport, the eight won by Soviet gymnast Aleksander Dityatin in 1980.

Phelps' performance was a welcome sporting story as news of two more drug cases emerged on day seven of the Olympics, which also saw the athletics competitions get into swing with two track gold medals up for grabs.

The International Olympic Committee (IOC) said Uzbekistan shot putter Olga Shchukina had been thrown out of the games after testing positive for a banned anabolic agent on August 14.

The 27-year-old, who finished last in Wednesday's shot put competition, had protested her innocence claiming to have ingested the drug accidentally in cough medicine. Details of the case were revealed by the IOC shortly after confirmation from Greek Olympic officials that one of their athletes, later identified by his coach as weightlifting bronze medallist Leonidas Sampanis, had failed a test.

A total of 12 athletes - eight weightlifters, two baseball players, one boxer and Shchukina - have now been confirmed as having failed drug tests by the IOC and sports officials since July 30.

Two more - Greek sprint stars Kostadinos Kenteris and Ekaterini Thanou - withdrew from the games after facing sanction for failing to take a drug test. IOC officials had forecast the possibility of record drug cases at this year's Olympics amid a sharp increase in controls and ever more sophisticated testing procedures.

World Anti-Doping Agency president Dick Pound told AFP in an interview the body would seek to widen its net to target coaches and sports officials. "There are very few accidental cases of doping. Most of them are organised programmes, designed by, implemented by, encouraged by, and condoned by coaches and sports officials" said Pound.

A new extension is to be made to the WADA doping control form that every athlete must sign. Within the next few months athletes will also have to list the name of their coach and doctor.

The drugs cases were threatening to cast a shadow over the opening day of the athletics which saw Italy's Ivano Brugnetti win the first of two track medals to be settled Friday at the 70,000-seater Olympic Stadium.

At the start of the day the United States were level with China on 14 gold medals each, with Japan third on nine golds. China increased their total to 15 by winning the women's doubles table tennis title, second seeds Wang Nan and Zhang Yining defeating South Korean third seeds Lee Eun-Sil and Seok Eun-Mi 11-9, 11-7, 11-6, 11-6.

The Chinese also guaranteed themselves another gold medal in the badminton tournament with Zhang Jiewen and Yang Wei advancing into an all-China final against Huang Sui and Gao Ling.

The US also won their 15th medal through marksman Matthew Emmons in the 50m prone rifle final before Phelps' win and a gold in the 50m freestyle swimming final from Gary Hall put the Americans on 17 medals.

Elsewhere on Friday, Great Britain won their first gold when flying Scot Chris Hoy claimed victory in the men's cycling one kilometre time trial. Britain are already assured of victory in sailing's Yngling event.

Anna Meares of Australia won the women's 500-metre time trial gold earlier in a new world record time of 33.952secs. Japan wrapped up the most successful Olympic judo campaign in their history with world open-weight champion Keiji Suzuki and women's 78kg star Maki Tsukada both winning gold medals on knockouts. -AFP

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