The deadly H5N1 bird flu virus has been found in poultry in a fourth Afghan province, a United Nations agency said on Tuesday, adding the disease seemed to be slowly spreading in the country.

Results of tests on 103 samples sent to Italy earlier this month, arrived on Monday and show that Kapisa province to the northeast of the capital, Kabul, is also affected by the disease.

The virus had been found already in samples from birds in Kabul, Logar and Nangarhar provinces.

No human cases have been found.

"According to the results, H5N1 Highly Pathogenic Avian Influenza (HPAI) cases have been definitely confirmed in four provinces: Kabul, Logar, Nangarhar and Kapisa," U.N. Food and Agriculture Organisation (FAO) said in a statement.

There are also strong suspicions that two other provinces -- Laghman and Parwan -- are affected but further analysis is needed, it said.

Landlocked Afghanistan, which has 34 provinces, lies at the junction of Central and South Asia and is on the migration route for several species of wild birds.

"Based on the currently available data, it seems that the disease is slowly spreading," the statement said. The FAO was backing efforts to strengthen surveillance systems.

Poultry has been culled and quarantine measures introduced in affected areas but the spread of the virus among poultry is a big worry in a country where many farmers and traders are illiterate and have little knowledge of the disease.

Scientists fear the virus could mutate into a form that jumps easily between people and start a global flu pandemic. While only 204 people are known to have been infected so far worldwide, 113 have died.

However, scientists say millions of people could be infected in a human pandemic but with a much lower mortality rate.