Russian officials traced dead poultry in several suburban Moscow districts to a single market Sunday as experts reported new outbreaks and tightened quarantines following confirmation of the presence of the H5N1 bird flu strain.

The presence of H5N1, confirmed by tests late Saturday, was the first such outbreak to be recorded so close to the Russian capital.

Authorities traced the birds that died in four separate incidences to a market located just outside the Moscow city limits, said Alexei Alexeyenko, spokesman for the federal agricultural oversight agency Rosselkhoznadzor.

The market was closed Saturday and experts were trying to determine the original source for the birds on sale there, he said.

On Saturday, Alexeyenko said tests had confirmed the H5N1 strain in some of the two dozen birds found dead in two suburban Moscow districts. Results of tests taken in a third district where nearly four dozen birds died were still pending, he said.

The Moscow region's chief veterinarian, Olga Gavrilenko, told Ekho Moskvy radio on Sunday that dead birds had also been reported in a forth district, north of the city.

Russian news agencies also reported a fifth incident of dead birds southwest of the capital, but Alexeyenko said it was too early to tell whether bird flu was to blame.

Officials said several people who were in close contact the dead poultry were taken to hospitals for observation, but no health problems had been reported.

Russian television broadcast footage showing veterinary workers clad in protective suits checking homes and spraying vehicle tires with disinfectant, while police began enforcing a quarantine in an effort to prevent the virus' spread.

Nikolai Vlasov, a senior Rosselkhoznadzor veterinary official, warned that more outbreaks were possible.

"If traces of the virus appear at the poultry market, we cannot exclude the possibility that there will be new outbreaks. But there should be no panic because conditions in Russia, especially in the winter, mean that there is not even any theoretical possibility of human infection," Vlasov said in televised comments.

No human cases of bird flu have been reported in Russia, which had its first reported cases of the H5N1 strain in birds in Siberia in 2005. Outbreaks have since occurred farther west, but mostly in southern areas distant from the capital.

Since it began ravaging Asian poultry farms in late 2003, the H5N1 strain has killed at least 167 people worldwide, according to the World Health Organization.

Though it remains difficult for humans to catch, health authorities across the globe are monitoring the H5N1 strain out of concern it could mutate into a form that easily spreads from person to person and spark a pandemic.