Members of the terrorist cell which carried out the attacks in Paris took advantage of the refugee crisis to "slip in" to France, according to the country's prime minister.
Manuel Valls said the EU's passport-free zone - under the Schengen agreement - would be "undermined" if Europe did not tighten security at its external borders.
And he warned authorities were unsure if there were any other groups or individual jihadists "still active", adding the terror threat would be "long and permanent".
His comments come ahead of an emergency meeting in Brussels called by French interior minister Bernard Cazeneuve, who urged the introduction of a passenger name record system to collect data on those who enter the EU.
Mr Valls told France 2: "These individuals took advantage of the refugee crisis ... of the chaos, perhaps, for some of them to slip in.
"The external borders of the European Union must be strengthened.
"If Europe does not assume its responsibilities, the whole Schengen system will be undermined."
Mr Valls spoke on French television after authorities confirmed the ringleader of last Friday's massacre, Abdelhamid Abaaoud, was among those killed as police besieged an apartment block in a Paris suburb.
Abaaoud, 27, had previously boasted of moving unrestricted between Syria and Belgium despite being flagged by intelligence agencies, while one of the suicide bombers who attacked the Stade de France was found to have used a Syrian passport in the name of Ahmad Al Mohammad to pass through Greece.
It was initially thought Belgian Abaaoud was in the Middle East before French police were tipped off by foreign intelligence that led them to the apartment in Saint-Denis on Wednesday.
The Islamic State (IS) militant was killed alongside his cousin Hasna Aitboulahcen, who blew herself up with a suicide vest.
Mr Valls admitted authorities "do not know" how Abaaoud entered France before the attacks on the nation's capital, which left 129 people dead.
Mr Cazeneuve earlier revealed the jihadist had returned from Syria in 2014 and had been involved in four of six foiled attacks in France since spring 2015.
And it is believed the extremists were set to carry out a second attack reportedly targeting Charles de Gaulle airport and the city's financial district La Defense before the pre-dawn raid on Wednesday.
Paris prosecutor Francois Molins said the operation neutralised a "new terrorist threat", and that "everything led us to believe that, considering their armaments, the structured organisation and their determination, they were ready to act".