France raised the risk level of avian influenza to 'high' from 'moderate' on Tuesday after the detection of new cases of the disease, forcing poultry farms to keep birds indoors to stem the spread of the highly contagious virus, reported Reuters.

The decision, taken by the agriculture ministry, was published in the Official Journal on Tuesday.

Avian influenza, commonly known as bird flu, has led to the culling of hundreds of millions birds worldwide in recent years. It usually strikes during autumn and winter and has been spreading in many European countries in the past weeks, including Germany, the Netherlands and Belgium.

France said last week that it had detected a first avian influenza outbreak on a farm this season in Brittany, in the northwest of the country.

The "high" risk level implies that all poultry should be kept inside on farms and additional security measures taken to avoid a spread of the disease.

Although the avian influenza is harmless in food, its spread is a concern for governments and the poultry industry due to the devastation it can cause to flocks, the possibility of trade restrictions and a risk of human transmission.

To counter the disease, France launched a vaccination campaign in early October, initially limited to ducks, which can easily transmit the virus without showing symptoms.