French anti-terror prosecutors have opened an investigation into two brothers suspected of plotting a "lethal and antisemitic" attack.

They say the men – described as Moroccan-Italian nationals aged 20 and 22 – were picked up last week in a car in northern France.

Officers found a loaded semi-automatic weapon, a bottle of hydrochloric acid and a flag of the Islamic State group in the vehicle, the prosecutors say.

As the current war in the Middle East enters its third week, many countries are on heightened alert for possible terror attacks motivated by the conflict. Jewish sites have been targeted with attacks in the US, the Netherlands and Belgium this week.

The pair are said to have arrived in France with their parents in 2017, after the family spent several years in Italy.

Anti-terrorist prosecutors said in a statement that, after concluding they could not wage jihad in Syria or the Palestinian territories, the brothers had in recent weeks developed a "terrorist plan in France, for which they aspired to martyrdom".

Analysis of seized digital devices, together with statements from relatives, revealed a "radicalisation of the two brothers over the past two years, as well as a marked intensification of their jihadist commitment in the days preceding their arrest", officials said.

The two brothers had taken steps in planning a "lethal and antisemitic" attack, the statement added, without giving specifics on the nature of the alleged plot.

There have been concerns that the threat of violence to Jewish communities around the world could be exacerbated due to the US-Israeli strikes on Iran.

An explosion at a Jewish school in Amsterdam on Saturday came a day after a suspected arson attack on a synagogue in Rotterdam the day prior.

In the US state of Michigan, a Lebanon-born gunman rammed ‌a pickup truck laden with fireworks and petrol into a synagogue on Thursday. The suspect killed himself, police said.