LYON (REUTERS/AFP) - At least three people were killed and more were gravely injured when an avalanche hit a school group and other skiers on a closed slope in the French Alps on Wednesday (Jan 13), the Interior Ministry said.

The avalanche hit a piste at the Deux-Alpes resort in eastern France that was still closed to the public, engulfing the group of French schoolchildren and their teacher from the city of Lyon, according to police sources.

A 14-year-old boy died in the deluge while a 16-year-old girl also died later in hospital. The Ukrainian man who died was not part of the school group.

The two teenagers were skiing as part of a group of 19 school pupils. Their teacher was seriously injured and taken to hospital.

All the remaining pupils were said to be safe, the French Education Ministry said in a statement.

Dozens of emergency workers earlier swarmed the black piste - the highest difficulty rating in France - while three search dogs hunted for any other possible victims.

Three helicopters hovered above the resort, one of them equipped with a thermal camera to try and track any survivors under the snow.

The Deux Alpes resort rises to an altitude of 3,600m and is around 60km from the city of Grenoble.

"The avalanche was 20 metres wide ... and 300 metres long," said Dominique Letang of the National Association for the Study of Snow and Avalanches (Anena).

"It was a typical case of a slab of snow formed by the heavy winds in recent days. Fresh snow did not attach enough" to the older layer, he added.

The skiing season got off to a slow start in France as unseasonably warm temperatures left slopes bare over the festive season with skiers and snowboarders having to make do with artificial snow.

However a sudden flurry of snow in recent days has covered the mountains, prompting the authorities to warn of a high risk of avalanches across the French Alps.

Four other people have died since January in avalanches in the French Alps - two Lithuanian mountain climbers, a Spaniard and a Czech.

"What is surprising is the number of people involved, even though we keep on saying that they must take it one at a time when the snow cover is unstable. It is infuriating," said Mr Letang.

"The avalanche risk was three on a scale of five. This piste was not open because there was not enough snow. It was not skiable," he added.

Dozens die each year in avalanches in France's popular ski resorts.

At least 45 people died in snowslides during the 2014/2015 winter season in France, according to the National Association for the Study of Snow and Avalanches, more than double the previous year.

Avalanches, which can see snow rushing down the mountainside at up to 200kmh, can be caused by several factors such as heavy rain, strong winds and the state of the soil beneath the snow.

As in this case, a significant fall of fresh snow which has not yet attached to the old layer is often to blame.

The deadliest avalanche in France's history took place in 1970 when 39 people where killed when their chalet was hit by an avalanche at the Val d'Isere ski resort.