An inquiry has been launched into the desecrations, AFP said
British soldiers' graves in a French military cemetery have been defaced with "Nazi graffiti", according to reports by the AFP news agency.
Swastikas and SS signs were daubed on the tombstones of about 12 soldiers killed in 1915, the agency reported.
The Loos-en-Gohelle cemetery in Pas-de-Calais has graves of 2,387 men killed at the battle of Loos in World War I.
French secretary of state for war veterans, Hubert Falco, said the act "insulted" the soldiers and France.
The AFP agency said an inquiry had been launched after the prefect of Pas-de-Calais, Pierre de Bousquet de Florian, visited the cemetery on Friday morning.
Mr Falco was quoted as saying the desecrations were "an insult to the memory of these soldiers who came to sacrifice themselves to defend France. They are also an insult to France."
He added that he "expressed his solidarity and friendship to his British and Canadian counterparts".
A spokesman for the Royal British Legion in London said: "It's self-evident that these graves must be respected and any acts of desecration are held in dismay by all right-thinking people."
The 11,000-sq-m cemetery holds the graves of 2,387 soldiers of the British Empire, including 39 Canadians, killed in the summer of 1915.