Police in France are increasing security at churches amid fears of an Islamist attack on the Feast of the Assumption.

Security had already been stepped up at France's 45,000 Catholic churches following the January terrorist shooting at the headquarters of satirical magazine Charlie Hebdo.

A letter from the country's Interior Ministry to Christians, published in France's Catholic daily newspaper La Croix, urged caution at services.

"Every religious gathering, whatever its magnitude, is vulnerable today to acts of malevolence, which may extend to terrorism," it said.

"Observing simple rules may allow you, without excessive distrust, to ensure your security and that of your community, while preserving the calm (and) sharing openness to others, which is needed for maintaining your faith and religious practices."

Churchgoers were encouraged to report 'very nervous' people and those who showed 'an unusual interest in the event or the building'.

Parishioners were told not to leave bags on pews as they went up to receive Communion.

In April three people were arrested by French police investigating an alleged plot to carry out a terrorist attack at a church near Paris.

Officers claim the three men were linked to a plan to kill Christians.