French President Francois Hollande gives a statement at the European Council headquarters in Brussels, June 26, 2015. [Photo/Agencies]

ISLAMIST FLAG

Police sources earlier said the decapitated body was found at the site, along with a flag bearing Islamist inscriptions.

Local newspaper Le Dauphine said the head covered in Arabic writing was found on a fence.

The French public prosecutor said its anti-terrorist section had been deployed to investigate.

France, which has deployed aircraft to the international coalition fighting Islamic State insurgents in Iraq, has long been named on Islamist sites as a primary target for attacks.

In April, Prime Minister Manuel Valls said no fewer than five attacks had been thwarted in the country since the Charlie Hebdo killings in January.

Then, Islamist gunmen killed 17 people in the offices of the Charlie Hebdo satirical weekly and a Jewish food store.

Noting that hundreds of French nationals are in Syria where they risked being radicalised by Islamist fighters, Valls said "never has the threat been so high".