Four people have been arrested in and around Montpellier, southern France, on suspicion of planning a terrorist attack, police and justice sources said on Friday.

Those in custody included a 22-year-old man and his 16 year-old girlfriend, both known to authorities for connections with radical Islam, after authorities found TATP explosives and other bomb-making materials in his home.

France remains on high alert over possible Islamist militant attacks. More than 230 people have died in a series of assaults since the beginning of 2015 and the country has been under state of emergency rules since November the same year.